[Qemu-devel] [PATCH v5 0/2] add read-pattern for block qourum

[Qemu-devel] [PATCH v5 0/2] add read-pattern for block qourum

[Liu Yuan]

v5:
 - simplify a for loop in quorum_aio_finalize()

v4:
 - swap the patch order
 - update comment for fifo pattern in qaip
 - use qapi enumeration in quorum driver instead of manual parsing

v3:
 - separate patch into two, one for quorum and one for qapi for easier review
 - add enumeration for quorum read pattern
 - remove unrelated blank line fix from this patch set

v2:
 - rename single as 'fifo'
 - rename read_all_children as read_quorum_children
 - fix quorum_aio_finalize() for fifo pattern

This patch adds single read pattern to quorum driver and quorum vote is default
pattern.

For now we do a quorum vote on all the reads, it is designed for unreliable
underlying storage such as non-redundant NFS to make sure data integrity at the
cost of the read performance.

For some use cases as following:

        VM
  --------------
  |            |
  v            v
  A            B

Both A and B has hardware raid storage to justify the data integrity on its own.
So it would help performance if we do a single read instead of on all the nodes.
Further, if we run VM on either of the storage node, we can make a local read
request for better performance.

This patch generalize the above 2 nodes case in the N nodes. That is,

vm -> write to all the N nodes, read just one of them. If single read fails, we
try to read next node in FIFO order specified by the startup command.

The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
functionality in the single device/node failure for now. But compared with DRBD
we still have some advantages over it:

- Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
storage. And if A crashes, we need to restart all the VMs on node B. But for
practice case, we can't because B might not have enough resources to setup 20 VMs
at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
images over the data center, we can very likely restart 20 VMs without any
resource problem.

After all, I think we can build a more powerful replicated image functionality
on quorum and block jobs(block mirror) to meet various High Availibility needs.

E.g, Enable single read pattern on 2 children,
  
-drive driver=quorum,children.0.file.filename=0.qcow2,\
children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1

[1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device

Cc: Benoit Canet <benoit@irqsave.net>
Cc: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Liu Yuan (2):
  qapi: add read-pattern enum for quorum
  block/quorum: add simple read pattern support

 block/quorum.c       | 176 +++++++++++++++++++++++++++++++++++++--------------
 qapi/block-core.json |  20 +++++-
 2 files changed, 148 insertions(+), 48 deletions(-)

-- 
1.9.1

[Qemu-devel] [PATCH v5 1/2] qapi: add read-pattern enum for quorum

[Liu Yuan]

Cc: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 qapi/block-core.json | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index e378653..42033d9 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1384,6 +1384,19 @@
             'raw': 'BlockdevRef' } }
 
 ##
+# @QuorumReadPattern
+#
+# An enumeration of quorum read patterns.
+#
+# @quorum: read all the children and do a quorum vote on reads
+#
+# @fifo: read only from the first child that has not failed
+#
+# Since: 2.2
+##
+{ 'enum': 'QuorumReadPattern', 'data': [ 'quorum', 'fifo' ] }
+
+##
 # @BlockdevOptionsQuorum
 #
 # Driver specific block device options for Quorum
@@ -1398,12 +1411,17 @@
 # @rewrite-corrupted: #optional rewrite corrupted data when quorum is reached
 #                     (Since 2.1)
 #
+# @read-pattern: #optional choose read pattern and set to quorum by default
+#                (Since 2.2)
+#
 # Since: 2.0
 ##
 { 'type': 'BlockdevOptionsQuorum',
   'data': { '*blkverify': 'bool',
             'children': [ 'BlockdevRef' ],
-            'vote-threshold': 'int', '*rewrite-corrupted': 'bool' } }
+            'vote-threshold': 'int',
+            '*rewrite-corrupted': 'bool',
+            '*read-pattern': 'QuorumReadPattern' } }
 
 ##
 # @BlockdevOptions
-- 
1.9.1

[Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Liu Yuan]

This patch adds single read pattern to quorum driver and quorum vote is default
pattern.

For now we do a quorum vote on all the reads, it is designed for unreliable
underlying storage such as non-redundant NFS to make sure data integrity at the
cost of the read performance.

For some use cases as following:

        VM
  --------------
  |            |
  v            v
  A            B

Both A and B has hardware raid storage to justify the data integrity on its own.
So it would help performance if we do a single read instead of on all the nodes.
Further, if we run VM on either of the storage node, we can make a local read
request for better performance.

This patch generalize the above 2 nodes case in the N nodes. That is,

vm -> write to all the N nodes, read just one of them. If single read fails, we
try to read next node in FIFO order specified by the startup command.

The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
functionality in the single device/node failure for now. But compared with DRBD
we still have some advantages over it:

- Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
storage. And if A crashes, we need to restart all the VMs on node B. But for
practice case, we can't because B might not have enough resources to setup 20 VMs
at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
images over the data center, we can very likely restart 20 VMs without any
resource problem.

After all, I think we can build a more powerful replicated image functionality
on quorum and block jobs(block mirror) to meet various High Availibility needs.

E.g, Enable single read pattern on 2 children,

-drive driver=quorum,children.0.file.filename=0.qcow2,\
children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1

[1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device

Cc: Benoit Canet <benoit@irqsave.net>
Cc: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
---
 block/quorum.c | 176 ++++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 129 insertions(+), 47 deletions(-)

diff --git a/block/quorum.c b/block/quorum.c
index d5ee9c0..1235d7c 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -24,6 +24,7 @@
 #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
 #define QUORUM_OPT_BLKVERIFY      "blkverify"
 #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
+#define QUORUM_OPT_READ_PATTERN   "read-pattern"
 
 /* This union holds a vote hash value */
 typedef union QuorumVoteValue {
@@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
     bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
                             * block if Quorum is reached.
                             */
+
+    QuorumReadPattern read_pattern;
 } BDRVQuorumState;
 
 typedef struct QuorumAIOCB QuorumAIOCB;
@@ -117,6 +120,7 @@ struct QuorumAIOCB {
 
     bool is_read;
     int vote_ret;
+    int child_iter;             /* which child to read in fifo pattern */
 };
 
 static bool quorum_vote(QuorumAIOCB *acb);
@@ -153,7 +157,8 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
     acb->common.cb(acb->common.opaque, ret);
 
     if (acb->is_read) {
-        for (i = 0; i < s->num_children; i++) {
+        /* on the quorum case acb->child_iter == s->num_children - 1 */
+        for (i = 0; i <= acb->child_iter; i++) {
             qemu_vfree(acb->qcrs[i].buf);
             qemu_iovec_destroy(&acb->qcrs[i].qiov);
         }
@@ -256,6 +261,21 @@ static void quorum_rewrite_aio_cb(void *opaque, int ret)
     quorum_aio_finalize(acb);
 }
 
+static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb);
+
+static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
+{
+    int i;
+    assert(dest->niov == source->niov);
+    assert(dest->size == source->size);
+    for (i = 0; i < source->niov; i++) {
+        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
+        memcpy(dest->iov[i].iov_base,
+               source->iov[i].iov_base,
+               source->iov[i].iov_len);
+    }
+}
+
 static void quorum_aio_cb(void *opaque, int ret)
 {
     QuorumChildRequest *sacb = opaque;
@@ -263,6 +283,21 @@ static void quorum_aio_cb(void *opaque, int ret)
     BDRVQuorumState *s = acb->common.bs->opaque;
     bool rewrite = false;
 
+    if (acb->is_read && s->read_pattern == QUORUM_READ_PATTERN_FIFO) {
+        /* We try to read next child in FIFO order if we fail to read */
+        if (ret < 0 && ++acb->child_iter < s->num_children) {
+            read_fifo_child(acb);
+            return;
+        }
+
+        if (ret == 0) {
+            quorum_copy_qiov(acb->qiov, &acb->qcrs[acb->child_iter].qiov);
+        }
+        acb->vote_ret = ret;
+        quorum_aio_finalize(acb);
+        return;
+    }
+
     sacb->ret = ret;
     acb->count++;
     if (ret == 0) {
@@ -343,19 +378,6 @@ static bool quorum_rewrite_bad_versions(BDRVQuorumState *s, QuorumAIOCB *acb,
     return count;
 }
 
-static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
-{
-    int i;
-    assert(dest->niov == source->niov);
-    assert(dest->size == source->size);
-    for (i = 0; i < source->niov; i++) {
-        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
-        memcpy(dest->iov[i].iov_base,
-               source->iov[i].iov_base,
-               source->iov[i].iov_len);
-    }
-}
-
 static void quorum_count_vote(QuorumVotes *votes,
                               QuorumVoteValue *value,
                               int index)
@@ -615,34 +637,62 @@ free_exit:
     return rewrite;
 }
 
-static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
-                                         int64_t sector_num,
-                                         QEMUIOVector *qiov,
-                                         int nb_sectors,
-                                         BlockDriverCompletionFunc *cb,
-                                         void *opaque)
+static BlockDriverAIOCB *read_quorum_children(QuorumAIOCB *acb)
 {
-    BDRVQuorumState *s = bs->opaque;
-    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
-                                      nb_sectors, cb, opaque);
+    BDRVQuorumState *s = acb->common.bs->opaque;
     int i;
 
-    acb->is_read = true;
-
     for (i = 0; i < s->num_children; i++) {
-        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], qiov->size);
-        qemu_iovec_init(&acb->qcrs[i].qiov, qiov->niov);
-        qemu_iovec_clone(&acb->qcrs[i].qiov, qiov, acb->qcrs[i].buf);
+        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], acb->qiov->size);
+        qemu_iovec_init(&acb->qcrs[i].qiov, acb->qiov->niov);
+        qemu_iovec_clone(&acb->qcrs[i].qiov, acb->qiov, acb->qcrs[i].buf);
     }
 
     for (i = 0; i < s->num_children; i++) {
-        bdrv_aio_readv(s->bs[i], sector_num, &acb->qcrs[i].qiov, nb_sectors,
-                       quorum_aio_cb, &acb->qcrs[i]);
+        bdrv_aio_readv(s->bs[i], acb->sector_num, &acb->qcrs[i].qiov,
+                       acb->nb_sectors, quorum_aio_cb, &acb->qcrs[i]);
     }
 
     return &acb->common;
 }
 
+static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb)
+{
+    BDRVQuorumState *s = acb->common.bs->opaque;
+
+    acb->qcrs[acb->child_iter].buf = qemu_blockalign(s->bs[acb->child_iter],
+                                                     acb->qiov->size);
+    qemu_iovec_init(&acb->qcrs[acb->child_iter].qiov, acb->qiov->niov);
+    qemu_iovec_clone(&acb->qcrs[acb->child_iter].qiov, acb->qiov,
+                     acb->qcrs[acb->child_iter].buf);
+    bdrv_aio_readv(s->bs[acb->child_iter], acb->sector_num,
+                   &acb->qcrs[acb->child_iter].qiov, acb->nb_sectors,
+                   quorum_aio_cb, &acb->qcrs[acb->child_iter]);
+
+    return &acb->common;
+}
+
+static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
+                                          int64_t sector_num,
+                                          QEMUIOVector *qiov,
+                                          int nb_sectors,
+                                          BlockDriverCompletionFunc *cb,
+                                          void *opaque)
+{
+    BDRVQuorumState *s = bs->opaque;
+    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
+                                      nb_sectors, cb, opaque);
+    acb->is_read = true;
+
+    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
+        acb->child_iter = s->num_children - 1;
+        return read_quorum_children(acb);
+    }
+
+    acb->child_iter = 0;
+    return read_fifo_child(acb);
+}
+
 static BlockDriverAIOCB *quorum_aio_writev(BlockDriverState *bs,
                                           int64_t sector_num,
                                           QEMUIOVector *qiov,
@@ -782,10 +832,33 @@ static QemuOptsList quorum_runtime_opts = {
             .type = QEMU_OPT_BOOL,
             .help = "Rewrite corrupted block on read quorum",
         },
+        {
+            .name = QUORUM_OPT_READ_PATTERN,
+            .type = QEMU_OPT_STRING,
+            .help = "Allowed pattern: quorum, fifo. Quorum is default",
+        },
         { /* end of list */ }
     },
 };
 
+static int parse_read_pattern(const char *opt)
+{
+    int i;
+
+    if (!opt) {
+        /* Set quorum as default */
+        return QUORUM_READ_PATTERN_QUORUM;
+    }
+
+    for (i = 0; i < QUORUM_READ_PATTERN_MAX; i++) {
+        if (!strcmp(opt, QuorumReadPattern_lookup[i])) {
+            return i;
+        }
+    }
+
+    return -EINVAL;
+}
+
 static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
                        Error **errp)
 {
@@ -827,28 +900,37 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     s->threshold = qemu_opt_get_number(opts, QUORUM_OPT_VOTE_THRESHOLD, 0);
-
-    /* and validate it against s->num_children */
-    ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
+    ret = parse_read_pattern(qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN));
     if (ret < 0) {
+        error_setg(&local_err, "Please set read-pattern as fifo or quorum\n");
         goto exit;
     }
+    s->read_pattern = ret;
 
-    /* is the driver in blkverify mode */
-    if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
-        s->num_children == 2 && s->threshold == 2) {
-        s->is_blkverify = true;
-    } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
-        fprintf(stderr, "blkverify mode is set by setting blkverify=on "
-                "and using two files with vote_threshold=2\n");
-    }
+    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
+        /* and validate it against s->num_children */
+        ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
+        if (ret < 0) {
+            goto exit;
+        }
 
-    s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE, false);
-    if (s->rewrite_corrupted && s->is_blkverify) {
-        error_setg(&local_err,
-                   "rewrite-corrupted=on cannot be used with blkverify=on");
-        ret = -EINVAL;
-        goto exit;
+        /* is the driver in blkverify mode */
+        if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
+            s->num_children == 2 && s->threshold == 2) {
+            s->is_blkverify = true;
+        } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
+            fprintf(stderr, "blkverify mode is set by setting blkverify=on "
+                    "and using two files with vote_threshold=2\n");
+        }
+
+        s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE,
+                                                 false);
+        if (s->rewrite_corrupted && s->is_blkverify) {
+            error_setg(&local_err,
+                       "rewrite-corrupted=on cannot be used with blkverify=on");
+            ret = -EINVAL;
+            goto exit;
+        }
     }
 
     /* allocate the children BlockDriverState array */
-- 
1.9.1

Re: [Qemu-devel] [PATCH v5 1/2] qapi: add read-pattern enum for quorum

[Benoît Canet]

The Friday 15 Aug 2014 à 13:05:16 (+0800), Liu Yuan wrote :
> Cc: Eric Blake <eblake@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> ---
>  qapi/block-core.json | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index e378653..42033d9 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1384,6 +1384,19 @@
>              'raw': 'BlockdevRef' } }
>  
>  ##
> +# @QuorumReadPattern
> +#
> +# An enumeration of quorum read patterns.
> +#
> +# @quorum: read all the children and do a quorum vote on reads
> +#
> +# @fifo: read only from the first child that has not failed
> +#
> +# Since: 2.2
> +##
> +{ 'enum': 'QuorumReadPattern', 'data': [ 'quorum', 'fifo' ] }
> +
> +##
>  # @BlockdevOptionsQuorum
>  #
>  # Driver specific block device options for Quorum
> @@ -1398,12 +1411,17 @@
>  # @rewrite-corrupted: #optional rewrite corrupted data when quorum is reached
>  #                     (Since 2.1)
>  #
> +# @read-pattern: #optional choose read pattern and set to quorum by default
> +#                (Since 2.2)
> +#
>  # Since: 2.0
>  ##
>  { 'type': 'BlockdevOptionsQuorum',
>    'data': { '*blkverify': 'bool',
>              'children': [ 'BlockdevRef' ],
> -            'vote-threshold': 'int', '*rewrite-corrupted': 'bool' } }
> +            'vote-threshold': 'int',
> +            '*rewrite-corrupted': 'bool',
> +            '*read-pattern': 'QuorumReadPattern' } }
>  
>  ##
>  # @BlockdevOptions
> -- 
> 1.9.1
> 
> 
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>

Re: [Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Benoît Canet]

The Friday 15 Aug 2014 à 13:05:17 (+0800), Liu Yuan wrote :
> This patch adds single read pattern to quorum driver and quorum vote is default
> pattern.
> 
> For now we do a quorum vote on all the reads, it is designed for unreliable
> underlying storage such as non-redundant NFS to make sure data integrity at the
> cost of the read performance.
> 
> For some use cases as following:
> 
>         VM
>   --------------
>   |            |
>   v            v
>   A            B
> 
> Both A and B has hardware raid storage to justify the data integrity on its own.
> So it would help performance if we do a single read instead of on all the nodes.
> Further, if we run VM on either of the storage node, we can make a local read
> request for better performance.
> 
> This patch generalize the above 2 nodes case in the N nodes. That is,
> 
> vm -> write to all the N nodes, read just one of them. If single read fails, we
> try to read next node in FIFO order specified by the startup command.
> 
> The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> functionality in the single device/node failure for now. But compared with DRBD
> we still have some advantages over it:
> 
> - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> storage. And if A crashes, we need to restart all the VMs on node B. But for
> practice case, we can't because B might not have enough resources to setup 20 VMs
> at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> images over the data center, we can very likely restart 20 VMs without any
> resource problem.
> 
> After all, I think we can build a more powerful replicated image functionality
> on quorum and block jobs(block mirror) to meet various High Availibility needs.
> 
> E.g, Enable single read pattern on 2 children,
> 
> -drive driver=quorum,children.0.file.filename=0.qcow2,\
> children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> 
> [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> 
> Cc: Benoit Canet <benoit@irqsave.net>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> ---
>  block/quorum.c | 176 ++++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 129 insertions(+), 47 deletions(-)
> 
> diff --git a/block/quorum.c b/block/quorum.c
> index d5ee9c0..1235d7c 100644
> --- a/block/quorum.c
> +++ b/block/quorum.c
> @@ -24,6 +24,7 @@
>  #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
>  #define QUORUM_OPT_BLKVERIFY      "blkverify"
>  #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
> +#define QUORUM_OPT_READ_PATTERN   "read-pattern"
>  
>  /* This union holds a vote hash value */
>  typedef union QuorumVoteValue {
> @@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
>      bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
>                              * block if Quorum is reached.
>                              */
> +
> +    QuorumReadPattern read_pattern;
>  } BDRVQuorumState;
>  
>  typedef struct QuorumAIOCB QuorumAIOCB;
> @@ -117,6 +120,7 @@ struct QuorumAIOCB {
>  
>      bool is_read;
>      int vote_ret;
> +    int child_iter;             /* which child to read in fifo pattern */

I don't understand what "fifo pattern" could mean for a bunch of disk
as they are not forming a queue.

Maybe round-robin is more suitable but your code does not implement
round-robin since it will alway start from the first disk.

Your code is scanning the disks set it's a scan pattern.

That said is it a problem that the first disk will be accessed more often than the other ?
You will have to care to insert disks in different order on each QEMU to spread the load.

Shouldn't the code try to spread the load by circling on the disk like a real round robin pattern ?

Best regards

Benoît

>  };
>  
>  static bool quorum_vote(QuorumAIOCB *acb);
> @@ -153,7 +157,8 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
>      acb->common.cb(acb->common.opaque, ret);
>  
>      if (acb->is_read) {
> -        for (i = 0; i < s->num_children; i++) {
> +        /* on the quorum case acb->child_iter == s->num_children - 1 */
> +        for (i = 0; i <= acb->child_iter; i++) {
>              qemu_vfree(acb->qcrs[i].buf);
>              qemu_iovec_destroy(&acb->qcrs[i].qiov);
>          }
> @@ -256,6 +261,21 @@ static void quorum_rewrite_aio_cb(void *opaque, int ret)
>      quorum_aio_finalize(acb);
>  }
>  
> +static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb);
> +
> +static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> +{
> +    int i;
> +    assert(dest->niov == source->niov);
> +    assert(dest->size == source->size);
> +    for (i = 0; i < source->niov; i++) {
> +        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> +        memcpy(dest->iov[i].iov_base,
> +               source->iov[i].iov_base,
> +               source->iov[i].iov_len);
> +    }
> +}
> +
>  static void quorum_aio_cb(void *opaque, int ret)
>  {
>      QuorumChildRequest *sacb = opaque;
> @@ -263,6 +283,21 @@ static void quorum_aio_cb(void *opaque, int ret)
>      BDRVQuorumState *s = acb->common.bs->opaque;
>      bool rewrite = false;
>  
> +    if (acb->is_read && s->read_pattern == QUORUM_READ_PATTERN_FIFO) {
> +        /* We try to read next child in FIFO order if we fail to read */
> +        if (ret < 0 && ++acb->child_iter < s->num_children) {
> +            read_fifo_child(acb);
> +            return;
> +        }
> +
> +        if (ret == 0) {
> +            quorum_copy_qiov(acb->qiov, &acb->qcrs[acb->child_iter].qiov);
> +        }
> +        acb->vote_ret = ret;
> +        quorum_aio_finalize(acb);
> +        return;
> +    }
> +
>      sacb->ret = ret;
>      acb->count++;
>      if (ret == 0) {
> @@ -343,19 +378,6 @@ static bool quorum_rewrite_bad_versions(BDRVQuorumState *s, QuorumAIOCB *acb,
>      return count;
>  }
>  
> -static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> -{
> -    int i;
> -    assert(dest->niov == source->niov);
> -    assert(dest->size == source->size);
> -    for (i = 0; i < source->niov; i++) {
> -        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> -        memcpy(dest->iov[i].iov_base,
> -               source->iov[i].iov_base,
> -               source->iov[i].iov_len);
> -    }
> -}
> -
>  static void quorum_count_vote(QuorumVotes *votes,
>                                QuorumVoteValue *value,
>                                int index)
> @@ -615,34 +637,62 @@ free_exit:
>      return rewrite;
>  }
>  
> -static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> -                                         int64_t sector_num,
> -                                         QEMUIOVector *qiov,
> -                                         int nb_sectors,
> -                                         BlockDriverCompletionFunc *cb,
> -                                         void *opaque)
> +static BlockDriverAIOCB *read_quorum_children(QuorumAIOCB *acb)
>  {
> -    BDRVQuorumState *s = bs->opaque;
> -    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
> -                                      nb_sectors, cb, opaque);
> +    BDRVQuorumState *s = acb->common.bs->opaque;
>      int i;
>  
> -    acb->is_read = true;
> -
>      for (i = 0; i < s->num_children; i++) {
> -        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], qiov->size);
> -        qemu_iovec_init(&acb->qcrs[i].qiov, qiov->niov);
> -        qemu_iovec_clone(&acb->qcrs[i].qiov, qiov, acb->qcrs[i].buf);
> +        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], acb->qiov->size);
> +        qemu_iovec_init(&acb->qcrs[i].qiov, acb->qiov->niov);
> +        qemu_iovec_clone(&acb->qcrs[i].qiov, acb->qiov, acb->qcrs[i].buf);
>      }
>  
>      for (i = 0; i < s->num_children; i++) {
> -        bdrv_aio_readv(s->bs[i], sector_num, &acb->qcrs[i].qiov, nb_sectors,
> -                       quorum_aio_cb, &acb->qcrs[i]);
> +        bdrv_aio_readv(s->bs[i], acb->sector_num, &acb->qcrs[i].qiov,
> +                       acb->nb_sectors, quorum_aio_cb, &acb->qcrs[i]);
>      }
>  
>      return &acb->common;
>  }
>  
> +static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb)
> +{
> +    BDRVQuorumState *s = acb->common.bs->opaque;
> +
> +    acb->qcrs[acb->child_iter].buf = qemu_blockalign(s->bs[acb->child_iter],
> +                                                     acb->qiov->size);
> +    qemu_iovec_init(&acb->qcrs[acb->child_iter].qiov, acb->qiov->niov);
> +    qemu_iovec_clone(&acb->qcrs[acb->child_iter].qiov, acb->qiov,
> +                     acb->qcrs[acb->child_iter].buf);
> +    bdrv_aio_readv(s->bs[acb->child_iter], acb->sector_num,
> +                   &acb->qcrs[acb->child_iter].qiov, acb->nb_sectors,
> +                   quorum_aio_cb, &acb->qcrs[acb->child_iter]);
> +
> +    return &acb->common;
> +}
> +
> +static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> +                                          int64_t sector_num,
> +                                          QEMUIOVector *qiov,
> +                                          int nb_sectors,
> +                                          BlockDriverCompletionFunc *cb,
> +                                          void *opaque)
> +{
> +    BDRVQuorumState *s = bs->opaque;
> +    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
> +                                      nb_sectors, cb, opaque);
> +    acb->is_read = true;
> +
> +    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
> +        acb->child_iter = s->num_children - 1;
> +        return read_quorum_children(acb);
> +    }
> +
> +    acb->child_iter = 0;
> +    return read_fifo_child(acb);
> +}
> +
>  static BlockDriverAIOCB *quorum_aio_writev(BlockDriverState *bs,
>                                            int64_t sector_num,
>                                            QEMUIOVector *qiov,
> @@ -782,10 +832,33 @@ static QemuOptsList quorum_runtime_opts = {
>              .type = QEMU_OPT_BOOL,
>              .help = "Rewrite corrupted block on read quorum",
>          },
> +        {
> +            .name = QUORUM_OPT_READ_PATTERN,
> +            .type = QEMU_OPT_STRING,
> +            .help = "Allowed pattern: quorum, fifo. Quorum is default",
> +        },
>          { /* end of list */ }
>      },
>  };
>  
> +static int parse_read_pattern(const char *opt)
> +{
> +    int i;
> +
> +    if (!opt) {
> +        /* Set quorum as default */
> +        return QUORUM_READ_PATTERN_QUORUM;
> +    }
> +
> +    for (i = 0; i < QUORUM_READ_PATTERN_MAX; i++) {
> +        if (!strcmp(opt, QuorumReadPattern_lookup[i])) {
> +            return i;
> +        }
> +    }
> +
> +    return -EINVAL;
> +}
> +
>  static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>                         Error **errp)
>  {
> @@ -827,28 +900,37 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>      }
>  
>      s->threshold = qemu_opt_get_number(opts, QUORUM_OPT_VOTE_THRESHOLD, 0);
> -
> -    /* and validate it against s->num_children */
> -    ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
> +    ret = parse_read_pattern(qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN));
>      if (ret < 0) {
> +        error_setg(&local_err, "Please set read-pattern as fifo or quorum\n");
>          goto exit;
>      }
> +    s->read_pattern = ret;
>  
> -    /* is the driver in blkverify mode */
> -    if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
> -        s->num_children == 2 && s->threshold == 2) {
> -        s->is_blkverify = true;
> -    } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
> -        fprintf(stderr, "blkverify mode is set by setting blkverify=on "
> -                "and using two files with vote_threshold=2\n");
> -    }
> +    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
> +        /* and validate it against s->num_children */
> +        ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
> +        if (ret < 0) {
> +            goto exit;
> +        }
>  
> -    s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE, false);
> -    if (s->rewrite_corrupted && s->is_blkverify) {
> -        error_setg(&local_err,
> -                   "rewrite-corrupted=on cannot be used with blkverify=on");
> -        ret = -EINVAL;
> -        goto exit;
> +        /* is the driver in blkverify mode */
> +        if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
> +            s->num_children == 2 && s->threshold == 2) {
> +            s->is_blkverify = true;
> +        } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
> +            fprintf(stderr, "blkverify mode is set by setting blkverify=on "
> +                    "and using two files with vote_threshold=2\n");
> +        }
> +
> +        s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE,
> +                                                 false);
> +        if (s->rewrite_corrupted && s->is_blkverify) {
> +            error_setg(&local_err,
> +                       "rewrite-corrupted=on cannot be used with blkverify=on");
> +            ret = -EINVAL;
> +            goto exit;
> +        }
>      }
>  
>      /* allocate the children BlockDriverState array */
> -- 
> 1.9.1
> 

Re: [Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Liu Yuan]

On Fri, Aug 15, 2014 at 03:59:04PM +0200, Benoît Canet wrote:
> The Friday 15 Aug 2014 à 13:05:17 (+0800), Liu Yuan wrote :
> > This patch adds single read pattern to quorum driver and quorum vote is default
> > pattern.
> > 
> > For now we do a quorum vote on all the reads, it is designed for unreliable
> > underlying storage such as non-redundant NFS to make sure data integrity at the
> > cost of the read performance.
> > 
> > For some use cases as following:
> > 
> >         VM
> >   --------------
> >   |            |
> >   v            v
> >   A            B
> > 
> > Both A and B has hardware raid storage to justify the data integrity on its own.
> > So it would help performance if we do a single read instead of on all the nodes.
> > Further, if we run VM on either of the storage node, we can make a local read
> > request for better performance.
> > 
> > This patch generalize the above 2 nodes case in the N nodes. That is,
> > 
> > vm -> write to all the N nodes, read just one of them. If single read fails, we
> > try to read next node in FIFO order specified by the startup command.
> > 
> > The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> > functionality in the single device/node failure for now. But compared with DRBD
> > we still have some advantages over it:
> > 
> > - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> > storage. And if A crashes, we need to restart all the VMs on node B. But for
> > practice case, we can't because B might not have enough resources to setup 20 VMs
> > at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> > images over the data center, we can very likely restart 20 VMs without any
> > resource problem.
> > 
> > After all, I think we can build a more powerful replicated image functionality
> > on quorum and block jobs(block mirror) to meet various High Availibility needs.
> > 
> > E.g, Enable single read pattern on 2 children,
> > 
> > -drive driver=quorum,children.0.file.filename=0.qcow2,\
> > children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> > 
> > [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> > 
> > Cc: Benoit Canet <benoit@irqsave.net>
> > Cc: Eric Blake <eblake@redhat.com>
> > Cc: Kevin Wolf <kwolf@redhat.com>
> > Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> > ---
> >  block/quorum.c | 176 ++++++++++++++++++++++++++++++++++++++++++---------------
> >  1 file changed, 129 insertions(+), 47 deletions(-)
> > 
> > diff --git a/block/quorum.c b/block/quorum.c
> > index d5ee9c0..1235d7c 100644
> > --- a/block/quorum.c
> > +++ b/block/quorum.c
> > @@ -24,6 +24,7 @@
> >  #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
> >  #define QUORUM_OPT_BLKVERIFY      "blkverify"
> >  #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
> > +#define QUORUM_OPT_READ_PATTERN   "read-pattern"
> >  
> >  /* This union holds a vote hash value */
> >  typedef union QuorumVoteValue {
> > @@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
> >      bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
> >                              * block if Quorum is reached.
> >                              */
> > +
> > +    QuorumReadPattern read_pattern;
> >  } BDRVQuorumState;
> >  
> >  typedef struct QuorumAIOCB QuorumAIOCB;
> > @@ -117,6 +120,7 @@ struct QuorumAIOCB {
> >  
> >      bool is_read;
> >      int vote_ret;
> > +    int child_iter;             /* which child to read in fifo pattern */
> 
> I don't understand what "fifo pattern" could mean for a bunch of disk
> as they are not forming a queue.

Naming isn't 100% accurate but as in Eric's comment (see below), both FIFO and
Round-Robin can be used for two different patterns.

> Maybe round-robin is more suitable but your code does not implement
> round-robin since it will alway start from the first disk.
> 
> Your code is scanning the disks set it's a scan pattern.
> 
> That said is it a problem that the first disk will be accessed more often than the other ?

As my commit log documented, the purpose of the read pattern I added is to
speed up read against quorum original read pattern. And the use case is clear
(I hope so) and you can take DRBD as a good example for why we need it. Of
course we are far away from DRBD, which need a recovery logic after all kinds of
failures. My patch set can be taken as a prelimitary step to implement a DRBD
like service driver.

Eric previously commented on two read patterns that might be useful:

"Should we offer multiple modes in addition to 'quorum'?  For example, I
could see a difference between 'fifo' (favor read from the first quorum
member always, unless it fails, good when the first member is local and
other member is remote) and 'round-robin' (evenly distribute reads; each
read goes to the next available quorum member, good when all members are
equally distant)."

> You will have to care to insert disks in different order on each QEMU to spread the load.

This is another use case that my patch set didn't try to solve.

> Shouldn't the code try to spread the load by circling on the disk like a real round robin pattern ?

Probably not on my patch set, but we can add a yet another round robin pattern
if anyone is intrested.

Thanks
Yuan

Re: [Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Liu Yuan]

On Mon, Aug 18, 2014 at 01:59:28PM +0800, Liu Yuan wrote:
> On Fri, Aug 15, 2014 at 03:59:04PM +0200, Benoît Canet wrote:
> > The Friday 15 Aug 2014 à 13:05:17 (+0800), Liu Yuan wrote :
> > > This patch adds single read pattern to quorum driver and quorum vote is default
> > > pattern.
> > > 
> > > For now we do a quorum vote on all the reads, it is designed for unreliable
> > > underlying storage such as non-redundant NFS to make sure data integrity at the
> > > cost of the read performance.
> > > 
> > > For some use cases as following:
> > > 
> > >         VM
> > >   --------------
> > >   |            |
> > >   v            v
> > >   A            B
> > > 
> > > Both A and B has hardware raid storage to justify the data integrity on its own.
> > > So it would help performance if we do a single read instead of on all the nodes.
> > > Further, if we run VM on either of the storage node, we can make a local read
> > > request for better performance.
> > > 
> > > This patch generalize the above 2 nodes case in the N nodes. That is,
> > > 
> > > vm -> write to all the N nodes, read just one of them. If single read fails, we
> > > try to read next node in FIFO order specified by the startup command.
> > > 
> > > The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> > > functionality in the single device/node failure for now. But compared with DRBD
> > > we still have some advantages over it:
> > > 
> > > - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> > > storage. And if A crashes, we need to restart all the VMs on node B. But for
> > > practice case, we can't because B might not have enough resources to setup 20 VMs
> > > at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> > > images over the data center, we can very likely restart 20 VMs without any
> > > resource problem.
> > > 
> > > After all, I think we can build a more powerful replicated image functionality
> > > on quorum and block jobs(block mirror) to meet various High Availibility needs.
> > > 
> > > E.g, Enable single read pattern on 2 children,
> > > 
> > > -drive driver=quorum,children.0.file.filename=0.qcow2,\
> > > children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> > > 
> > > [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> > > 
> > > Cc: Benoit Canet <benoit@irqsave.net>
> > > Cc: Eric Blake <eblake@redhat.com>
> > > Cc: Kevin Wolf <kwolf@redhat.com>
> > > Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > > Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> > > ---
> > >  block/quorum.c | 176 ++++++++++++++++++++++++++++++++++++++++++---------------
> > >  1 file changed, 129 insertions(+), 47 deletions(-)
> > > 
> > > diff --git a/block/quorum.c b/block/quorum.c
> > > index d5ee9c0..1235d7c 100644
> > > --- a/block/quorum.c
> > > +++ b/block/quorum.c
> > > @@ -24,6 +24,7 @@
> > >  #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
> > >  #define QUORUM_OPT_BLKVERIFY      "blkverify"
> > >  #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
> > > +#define QUORUM_OPT_READ_PATTERN   "read-pattern"
> > >  
> > >  /* This union holds a vote hash value */
> > >  typedef union QuorumVoteValue {
> > > @@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
> > >      bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
> > >                              * block if Quorum is reached.
> > >                              */
> > > +
> > > +    QuorumReadPattern read_pattern;
> > >  } BDRVQuorumState;
> > >  
> > >  typedef struct QuorumAIOCB QuorumAIOCB;
> > > @@ -117,6 +120,7 @@ struct QuorumAIOCB {
> > >  
> > >      bool is_read;
> > >      int vote_ret;
> > > +    int child_iter;             /* which child to read in fifo pattern */
> > 
> > I don't understand what "fifo pattern" could mean for a bunch of disk
> > as they are not forming a queue.
> 
> Naming isn't 100% accurate but as in Eric's comment (see below), both FIFO and
> Round-Robin can be used for two different patterns.
> 
> > Maybe round-robin is more suitable but your code does not implement
> > round-robin since it will alway start from the first disk.
> > 
> > Your code is scanning the disks set it's a scan pattern.
> > 
> > That said is it a problem that the first disk will be accessed more often than the other ?
> 
> As my commit log documented, the purpose of the read pattern I added is to
> speed up read against quorum original read pattern. And the use case is clear
> (I hope so) and you can take DRBD as a good example for why we need it. Of
> course we are far away from DRBD, which need a recovery logic after all kinds of
> failures. My patch set can be taken as a prelimitary step to implement a DRBD
> like service driver.
> 
> Eric previously commented on two read patterns that might be useful:
> 
> "Should we offer multiple modes in addition to 'quorum'?  For example, I
> could see a difference between 'fifo' (favor read from the first quorum
> member always, unless it fails, good when the first member is local and
> other member is remote) and 'round-robin' (evenly distribute reads; each
> read goes to the next available quorum member, good when all members are
> equally distant)."
> 
> > You will have to care to insert disks in different order on each QEMU to spread the load.
> 
> This is another use case that my patch set didn't try to solve.
> 
> > Shouldn't the code try to spread the load by circling on the disk like a real round robin pattern ?
> 
> Probably not on my patch set, but we can add a yet another round robin pattern
> if anyone is intrested.

Benoit, ping...

Re: [Qemu-devel] [PATCH v5 2/2] block/quorum: add simple read pattern support

[Benoît Canet]

The Friday 15 Aug 2014 à 13:05:17 (+0800), Liu Yuan wrote :
> This patch adds single read pattern to quorum driver and quorum vote is default
> pattern.
> 
> For now we do a quorum vote on all the reads, it is designed for unreliable
> underlying storage such as non-redundant NFS to make sure data integrity at the
> cost of the read performance.
> 
> For some use cases as following:
> 
>         VM
>   --------------
>   |            |
>   v            v
>   A            B
> 
> Both A and B has hardware raid storage to justify the data integrity on its own.
> So it would help performance if we do a single read instead of on all the nodes.
> Further, if we run VM on either of the storage node, we can make a local read
> request for better performance.
> 
> This patch generalize the above 2 nodes case in the N nodes. That is,
> 
> vm -> write to all the N nodes, read just one of them. If single read fails, we
> try to read next node in FIFO order specified by the startup command.
> 
> The 2 nodes case is very similar to DRBD[1] though lack of auto-sync
> functionality in the single device/node failure for now. But compared with DRBD
> we still have some advantages over it:
> 
> - Suppose we have 20 VMs running on one(assume A) of 2 nodes' DRBD backed
> storage. And if A crashes, we need to restart all the VMs on node B. But for
> practice case, we can't because B might not have enough resources to setup 20 VMs
> at once. So if we run our 20 VMs with quorum driver, and scatter the replicated
> images over the data center, we can very likely restart 20 VMs without any
> resource problem.
> 
> After all, I think we can build a more powerful replicated image functionality
> on quorum and block jobs(block mirror) to meet various High Availibility needs.
> 
> E.g, Enable single read pattern on 2 children,
> 
> -drive driver=quorum,children.0.file.filename=0.qcow2,\
> children.1.file.filename=1.qcow2,read-pattern=fifo,vote-threshold=1
> 
> [1] http://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device
> 
> Cc: Benoit Canet <benoit@irqsave.net>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Liu Yuan <namei.unix@gmail.com>
> ---
>  block/quorum.c | 176 ++++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 129 insertions(+), 47 deletions(-)
> 
> diff --git a/block/quorum.c b/block/quorum.c
> index d5ee9c0..1235d7c 100644
> --- a/block/quorum.c
> +++ b/block/quorum.c
> @@ -24,6 +24,7 @@
>  #define QUORUM_OPT_VOTE_THRESHOLD "vote-threshold"
>  #define QUORUM_OPT_BLKVERIFY      "blkverify"
>  #define QUORUM_OPT_REWRITE        "rewrite-corrupted"
> +#define QUORUM_OPT_READ_PATTERN   "read-pattern"
>  
>  /* This union holds a vote hash value */
>  typedef union QuorumVoteValue {
> @@ -74,6 +75,8 @@ typedef struct BDRVQuorumState {
>      bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
>                              * block if Quorum is reached.
>                              */
> +
> +    QuorumReadPattern read_pattern;
>  } BDRVQuorumState;
>  
>  typedef struct QuorumAIOCB QuorumAIOCB;
> @@ -117,6 +120,7 @@ struct QuorumAIOCB {
>  
>      bool is_read;
>      int vote_ret;
> +    int child_iter;             /* which child to read in fifo pattern */
>  };
>  
>  static bool quorum_vote(QuorumAIOCB *acb);
> @@ -153,7 +157,8 @@ static void quorum_aio_finalize(QuorumAIOCB *acb)
>      acb->common.cb(acb->common.opaque, ret);
>  
>      if (acb->is_read) {
> -        for (i = 0; i < s->num_children; i++) {
> +        /* on the quorum case acb->child_iter == s->num_children - 1 */
> +        for (i = 0; i <= acb->child_iter; i++) {
>              qemu_vfree(acb->qcrs[i].buf);
>              qemu_iovec_destroy(&acb->qcrs[i].qiov);
>          }
> @@ -256,6 +261,21 @@ static void quorum_rewrite_aio_cb(void *opaque, int ret)
>      quorum_aio_finalize(acb);
>  }
>  
> +static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb);
> +
> +static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> +{
> +    int i;
> +    assert(dest->niov == source->niov);
> +    assert(dest->size == source->size);
> +    for (i = 0; i < source->niov; i++) {
> +        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> +        memcpy(dest->iov[i].iov_base,
> +               source->iov[i].iov_base,
> +               source->iov[i].iov_len);
> +    }
> +}
> +
>  static void quorum_aio_cb(void *opaque, int ret)
>  {
>      QuorumChildRequest *sacb = opaque;
> @@ -263,6 +283,21 @@ static void quorum_aio_cb(void *opaque, int ret)
>      BDRVQuorumState *s = acb->common.bs->opaque;
>      bool rewrite = false;
>  
> +    if (acb->is_read && s->read_pattern == QUORUM_READ_PATTERN_FIFO) {
> +        /* We try to read next child in FIFO order if we fail to read */
> +        if (ret < 0 && ++acb->child_iter < s->num_children) {
> +            read_fifo_child(acb);
> +            return;
> +        }
> +
> +        if (ret == 0) {
> +            quorum_copy_qiov(acb->qiov, &acb->qcrs[acb->child_iter].qiov);
> +        }
> +        acb->vote_ret = ret;
> +        quorum_aio_finalize(acb);
> +        return;
> +    }
> +
>      sacb->ret = ret;
>      acb->count++;
>      if (ret == 0) {
> @@ -343,19 +378,6 @@ static bool quorum_rewrite_bad_versions(BDRVQuorumState *s, QuorumAIOCB *acb,
>      return count;
>  }
>  
> -static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> -{
> -    int i;
> -    assert(dest->niov == source->niov);
> -    assert(dest->size == source->size);
> -    for (i = 0; i < source->niov; i++) {
> -        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> -        memcpy(dest->iov[i].iov_base,
> -               source->iov[i].iov_base,
> -               source->iov[i].iov_len);
> -    }
> -}
> -
>  static void quorum_count_vote(QuorumVotes *votes,
>                                QuorumVoteValue *value,
>                                int index)
> @@ -615,34 +637,62 @@ free_exit:
>      return rewrite;
>  }
>  
> -static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> -                                         int64_t sector_num,
> -                                         QEMUIOVector *qiov,
> -                                         int nb_sectors,
> -                                         BlockDriverCompletionFunc *cb,
> -                                         void *opaque)
> +static BlockDriverAIOCB *read_quorum_children(QuorumAIOCB *acb)
>  {
> -    BDRVQuorumState *s = bs->opaque;
> -    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
> -                                      nb_sectors, cb, opaque);
> +    BDRVQuorumState *s = acb->common.bs->opaque;
>      int i;
>  
> -    acb->is_read = true;
> -
>      for (i = 0; i < s->num_children; i++) {
> -        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], qiov->size);
> -        qemu_iovec_init(&acb->qcrs[i].qiov, qiov->niov);
> -        qemu_iovec_clone(&acb->qcrs[i].qiov, qiov, acb->qcrs[i].buf);
> +        acb->qcrs[i].buf = qemu_blockalign(s->bs[i], acb->qiov->size);
> +        qemu_iovec_init(&acb->qcrs[i].qiov, acb->qiov->niov);
> +        qemu_iovec_clone(&acb->qcrs[i].qiov, acb->qiov, acb->qcrs[i].buf);
>      }
>  
>      for (i = 0; i < s->num_children; i++) {
> -        bdrv_aio_readv(s->bs[i], sector_num, &acb->qcrs[i].qiov, nb_sectors,
> -                       quorum_aio_cb, &acb->qcrs[i]);
> +        bdrv_aio_readv(s->bs[i], acb->sector_num, &acb->qcrs[i].qiov,
> +                       acb->nb_sectors, quorum_aio_cb, &acb->qcrs[i]);
>      }
>  
>      return &acb->common;
>  }
>  
> +static BlockDriverAIOCB *read_fifo_child(QuorumAIOCB *acb)
> +{
> +    BDRVQuorumState *s = acb->common.bs->opaque;
> +
> +    acb->qcrs[acb->child_iter].buf = qemu_blockalign(s->bs[acb->child_iter],
> +                                                     acb->qiov->size);
> +    qemu_iovec_init(&acb->qcrs[acb->child_iter].qiov, acb->qiov->niov);
> +    qemu_iovec_clone(&acb->qcrs[acb->child_iter].qiov, acb->qiov,
> +                     acb->qcrs[acb->child_iter].buf);
> +    bdrv_aio_readv(s->bs[acb->child_iter], acb->sector_num,
> +                   &acb->qcrs[acb->child_iter].qiov, acb->nb_sectors,
> +                   quorum_aio_cb, &acb->qcrs[acb->child_iter]);
> +
> +    return &acb->common;
> +}
> +
> +static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
> +                                          int64_t sector_num,
> +                                          QEMUIOVector *qiov,
> +                                          int nb_sectors,
> +                                          BlockDriverCompletionFunc *cb,
> +                                          void *opaque)
> +{
> +    BDRVQuorumState *s = bs->opaque;
> +    QuorumAIOCB *acb = quorum_aio_get(s, bs, qiov, sector_num,
> +                                      nb_sectors, cb, opaque);
> +    acb->is_read = true;
> +
> +    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
> +        acb->child_iter = s->num_children - 1;
> +        return read_quorum_children(acb);
> +    }
> +
> +    acb->child_iter = 0;
> +    return read_fifo_child(acb);
> +}
> +
>  static BlockDriverAIOCB *quorum_aio_writev(BlockDriverState *bs,
>                                            int64_t sector_num,
>                                            QEMUIOVector *qiov,
> @@ -782,10 +832,33 @@ static QemuOptsList quorum_runtime_opts = {
>              .type = QEMU_OPT_BOOL,
>              .help = "Rewrite corrupted block on read quorum",
>          },
> +        {
> +            .name = QUORUM_OPT_READ_PATTERN,
> +            .type = QEMU_OPT_STRING,
> +            .help = "Allowed pattern: quorum, fifo. Quorum is default",
> +        },
>          { /* end of list */ }
>      },
>  };
>  
> +static int parse_read_pattern(const char *opt)
> +{
> +    int i;
> +
> +    if (!opt) {
> +        /* Set quorum as default */
> +        return QUORUM_READ_PATTERN_QUORUM;
> +    }
> +
> +    for (i = 0; i < QUORUM_READ_PATTERN_MAX; i++) {
> +        if (!strcmp(opt, QuorumReadPattern_lookup[i])) {
> +            return i;
> +        }
> +    }
> +
> +    return -EINVAL;
> +}
> +
>  static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>                         Error **errp)
>  {
> @@ -827,28 +900,37 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>      }
>  
>      s->threshold = qemu_opt_get_number(opts, QUORUM_OPT_VOTE_THRESHOLD, 0);
> -
> -    /* and validate it against s->num_children */
> -    ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
> +    ret = parse_read_pattern(qemu_opt_get(opts, QUORUM_OPT_READ_PATTERN));
>      if (ret < 0) {
> +        error_setg(&local_err, "Please set read-pattern as fifo or quorum\n");
>          goto exit;
>      }
> +    s->read_pattern = ret;
>  
> -    /* is the driver in blkverify mode */
> -    if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
> -        s->num_children == 2 && s->threshold == 2) {
> -        s->is_blkverify = true;
> -    } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
> -        fprintf(stderr, "blkverify mode is set by setting blkverify=on "
> -                "and using two files with vote_threshold=2\n");
> -    }
> +    if (s->read_pattern == QUORUM_READ_PATTERN_QUORUM) {
> +        /* and validate it against s->num_children */
> +        ret = quorum_valid_threshold(s->threshold, s->num_children, &local_err);
> +        if (ret < 0) {
> +            goto exit;
> +        }
>  
> -    s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE, false);
> -    if (s->rewrite_corrupted && s->is_blkverify) {
> -        error_setg(&local_err,
> -                   "rewrite-corrupted=on cannot be used with blkverify=on");
> -        ret = -EINVAL;
> -        goto exit;
> +        /* is the driver in blkverify mode */
> +        if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false) &&
> +            s->num_children == 2 && s->threshold == 2) {
> +            s->is_blkverify = true;
> +        } else if (qemu_opt_get_bool(opts, QUORUM_OPT_BLKVERIFY, false)) {
> +            fprintf(stderr, "blkverify mode is set by setting blkverify=on "
> +                    "and using two files with vote_threshold=2\n");
> +        }
> +
> +        s->rewrite_corrupted = qemu_opt_get_bool(opts, QUORUM_OPT_REWRITE,
> +                                                 false);
> +        if (s->rewrite_corrupted && s->is_blkverify) {
> +            error_setg(&local_err,
> +                       "rewrite-corrupted=on cannot be used with blkverify=on");
> +            ret = -EINVAL;
> +            goto exit;
> +        }
>      }
>  
>      /* allocate the children BlockDriverState array */
> -- 
> 1.9.1
> 

Reviewed-by: Benoit Canet <benoit.canet@nodalink.com>