[Qemu-devel] [PATCH v5 0/3] qcow2 metadata cache

[Qemu-devel] [PATCH v5 0/3] qcow2 metadata cache

[Kevin Wolf]

block-queue turned out to be too big effort to be useful for quickly fixing the
performance problems that qcow2 got since we introduced the metadata flushes.
While I still think the idea is right, it needs more time and qcow2 doesn't
have more time. Let's come back to block-queue later when the most urgent qcow2
problems are fixed.

So this is the idea of block-queue applied to the very specific case of qcow2.
Whereas block-queue tried to be a generic solution for all kind of things and
tried to make all writes asynchronous at the same time, this is only about
batching writes to refcount blocks and L2 tables in qcow2 and getting the
dependencies right. (Yes, the L1 table and refcount table is left alone. They
are almost never written to anyway.)

This should be much easier to understand and review, and I myself feel a bit
more confident about it than with block-queue, too.

v1:
- Don't read newly allocated tables from the disk before memsetting them to
  zero

v2:
- Addressed Stefan's review comments
- Added patch 3 to avoid an unnecessary bdrv_flush after COW

v3:
- Some error path fixes (esp. missing or double qcow2_cache_put)

v4:
- Pay attention to make writethrough performance not even worse in
  update_refcount
- Add some blkdebug events back so that "failures" in qemu-iotests are kept
  minimal. Some diff between master and this series is left in test 026, but
  it's harmless.

v5:
- Fixed return value of alloc_refcount_block
- Removed unnecessary bdrv_flush

Kevin Wolf (3):
  qcow2: Add QcowCache
  qcow2: Use QcowCache
  qcow2: Batch flushes for COW

 Makefile.objs          |    2 +-
 block/qcow2-cache.c    |  314 ++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2-cluster.c  |  208 +++++++++++---------------------
 block/qcow2-refcount.c |  260 ++++++++++++++++-----------------------
 block/qcow2.c          |   48 +++++++-
 block/qcow2.h          |   32 ++++-
 6 files changed, 565 insertions(+), 299 deletions(-)
 create mode 100644 block/qcow2-cache.c

-- 
1.7.2.3

[Qemu-devel] [PATCH v5 1/3] qcow2: Add QcowCache

[Kevin Wolf]

This adds some new cache functions to qcow2 which can be used for caching
refcount blocks and L2 tables. When used with cache=writethrough they work
like the old caching code which is spread all over qcow2, so for this case we
have merely a cleanup.

The interesting case is with writeback caching (this includes cache=none) where
data isn't written to disk immediately but only kept in cache initially. This
leads to some form of metadata write batching which avoids the current "write
to refcount block, flush, write to L2 table" pattern for each single request
when a lot of cluster allocations happen. Instead, cache entries are only
written out if its required to maintain the right order. In the pure cluster
allocation case this means that all metadata updates for requests are done in
memory initially and on sync, first the refcount blocks are written to disk,
then fsync, then L2 tables.

This improves performance of scenarios with lots of cluster allocations
noticably (e.g. installation or after taking a snapshot).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 Makefile.objs       |    2 +-
 block/qcow2-cache.c |  290 +++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.h       |   19 ++++
 3 files changed, 310 insertions(+), 1 deletions(-)
 create mode 100644 block/qcow2-cache.c

diff --git a/Makefile.objs b/Makefile.objs
index fda366d..93406ff 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -19,7 +19,7 @@ block-obj-$(CONFIG_POSIX) += posix-aio-compat.o
 block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
 
 block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
-block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o
+block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
 block-nested-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
 block-nested-y += qed-check.o
 block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
new file mode 100644
index 0000000..f7c4e2a
--- /dev/null
+++ b/block/qcow2-cache.c
@@ -0,0 +1,290 @@
+/*
+ * L2/refcount table cache for the QCOW2 format
+ *
+ * Copyright (c) 2010 Kevin Wolf <kwolf@redhat.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "block_int.h"
+#include "qemu-common.h"
+#include "qcow2.h"
+
+typedef struct Qcow2CachedTable {
+    void*   table;
+    int64_t offset;
+    bool    dirty;
+    int     cache_hits;
+    int     ref;
+} Qcow2CachedTable;
+
+struct Qcow2Cache {
+    int                     size;
+    Qcow2CachedTable*       entries;
+    struct Qcow2Cache*      depends;
+    bool                    writethrough;
+};
+
+Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
+    bool writethrough)
+{
+    BDRVQcowState *s = bs->opaque;
+    Qcow2Cache *c;
+    int i;
+
+    c = qemu_mallocz(sizeof(*c));
+    c->size = num_tables;
+    c->entries = qemu_mallocz(sizeof(*c->entries) * num_tables);
+    c->writethrough = writethrough;
+
+    for (i = 0; i < c->size; i++) {
+        c->entries[i].table = qemu_blockalign(bs, s->cluster_size);
+    }
+
+    return c;
+}
+
+int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c)
+{
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        assert(c->entries[i].ref == 0);
+        qemu_vfree(c->entries[i].table);
+    }
+
+    qemu_free(c->entries);
+    qemu_free(c);
+
+    return 0;
+}
+
+static int qcow2_cache_flush_dependency(BlockDriverState *bs, Qcow2Cache *c)
+{
+    int ret;
+
+    ret = qcow2_cache_flush(bs, c->depends);
+    if (ret < 0) {
+        return ret;
+    }
+
+    c->depends = NULL;
+    return 0;
+}
+
+static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+
+    if (!c->entries[i].dirty || !c->entries[i].offset) {
+        return 0;
+    }
+
+    if (c->depends) {
+        ret = qcow2_cache_flush_dependency(bs, c);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    ret = bdrv_pwrite(bs->file, c->entries[i].offset, c->entries[i].table,
+        s->cluster_size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    c->entries[i].dirty = false;
+
+    return 0;
+}
+
+int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
+{
+    int result = 0;
+    int ret;
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        ret = qcow2_cache_entry_flush(bs, c, i);
+        if (ret < 0 && result != -ENOSPC) {
+            result = ret;
+        }
+    }
+
+    if (result == 0) {
+        ret = bdrv_flush(bs->file);
+        if (ret < 0) {
+            result = ret;
+        }
+    }
+
+    return result;
+}
+
+int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
+    Qcow2Cache *dependency)
+{
+    int ret;
+
+    if (dependency->depends) {
+        ret = qcow2_cache_flush_dependency(bs, dependency);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (c->depends && (c->depends != dependency)) {
+        ret = qcow2_cache_flush_dependency(bs, c);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    c->depends = dependency;
+    return 0;
+}
+
+static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
+{
+    int i;
+    int min_count = INT_MAX;
+    int min_index = -1;
+
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].ref) {
+            continue;
+        }
+
+        if (c->entries[i].cache_hits < min_count) {
+            min_index = i;
+            min_count = c->entries[i].cache_hits;
+        }
+
+        /* Give newer hits priority */
+        /* TODO Check how to optimize the replacement strategy */
+        c->entries[i].cache_hits /= 2;
+    }
+
+    if (min_index == -1) {
+        /* This can't happen in current synchronous code, but leave the check
+         * here as a reminder for whoever starts using AIO with the cache */
+        abort();
+    }
+    return min_index;
+}
+
+static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
+    uint64_t offset, void **table, bool read_from_disk)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i;
+    int ret;
+
+    /* Check if the table is already cached */
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].offset == offset) {
+            goto found;
+        }
+    }
+
+    /* If not, write a table back and replace it */
+    i = qcow2_cache_find_entry_to_replace(c);
+    if (i < 0) {
+        return i;
+    }
+
+    ret = qcow2_cache_entry_flush(bs, c, i);
+    if (ret < 0) {
+        return ret;
+    }
+
+    c->entries[i].offset = 0;
+    if (read_from_disk) {
+        ret = bdrv_pread(bs->file, offset, c->entries[i].table, s->cluster_size);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    /* Give the table some hits for the start so that it won't be replaced
+     * immediately. The number 32 is completely arbitrary. */
+    c->entries[i].cache_hits = 32;
+    c->entries[i].offset = offset;
+
+    /* And return the right table */
+found:
+    c->entries[i].cache_hits++;
+    c->entries[i].ref++;
+    *table = c->entries[i].table;
+    return 0;
+}
+
+int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table)
+{
+    return qcow2_cache_do_get(bs, c, offset, table, true);
+}
+
+int qcow2_cache_get_empty(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table)
+{
+    return qcow2_cache_do_get(bs, c, offset, table, false);
+}
+
+int qcow2_cache_put(BlockDriverState *bs, Qcow2Cache *c, void **table)
+{
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].table == *table) {
+            goto found;
+        }
+    }
+    return -ENOENT;
+
+found:
+    c->entries[i].ref--;
+    *table = NULL;
+
+    assert(c->entries[i].ref >= 0);
+
+    if (c->writethrough) {
+        return qcow2_cache_entry_flush(bs, c, i);
+    } else {
+        return 0;
+    }
+}
+
+void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table)
+{
+    int i;
+
+    for (i = 0; i < c->size; i++) {
+        if (c->entries[i].table == table) {
+            goto found;
+        }
+    }
+    abort();
+
+found:
+    c->entries[i].dirty = true;
+}
+
diff --git a/block/qcow2.h b/block/qcow2.h
index 5217bea..e5473e1 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -78,6 +78,9 @@ typedef struct QCowSnapshot {
     uint64_t vm_clock_nsec;
 } QCowSnapshot;
 
+struct Qcow2Cache;
+typedef struct Qcow2Cache Qcow2Cache;
+
 typedef struct BDRVQcowState {
     int cluster_bits;
     int cluster_size;
@@ -215,4 +218,20 @@ int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name);
 void qcow2_free_snapshots(BlockDriverState *bs);
 int qcow2_read_snapshots(BlockDriverState *bs);
 
+/* qcow2-cache.c functions */
+Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
+    bool writethrough);
+int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c);
+
+void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
+int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
+int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
+    Qcow2Cache *dependency);
+
+int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table);
+int qcow2_cache_get_empty(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table);
+int qcow2_cache_put(BlockDriverState *bs, Qcow2Cache *c, void **table);
+
 #endif
-- 
1.7.2.3

[Qemu-devel] [PATCH v5 2/3] qcow2: Use QcowCache

[Kevin Wolf]

Use the new functions of qcow2-cache.c for everything that works on refcount
block and L2 tables.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cache.c    |   10 ++
 block/qcow2-cluster.c  |  208 +++++++++++++-------------------------
 block/qcow2-refcount.c |  260 ++++++++++++++++++++----------------------------
 block/qcow2.c          |   48 ++++++++-
 block/qcow2.h          |   12 ++-
 5 files changed, 240 insertions(+), 298 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index f7c4e2a..5f1740b 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -104,6 +104,12 @@ static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
         }
     }
 
+    if (c == s->refcount_block_cache) {
+        BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_UPDATE_PART);
+    } else if (c == s->l2_table_cache) {
+        BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE);
+    }
+
     ret = bdrv_pwrite(bs->file, c->entries[i].offset, c->entries[i].table,
         s->cluster_size);
     if (ret < 0) {
@@ -218,6 +224,10 @@ static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
 
     c->entries[i].offset = 0;
     if (read_from_disk) {
+        if (c == s->l2_table_cache) {
+            BLKDBG_EVENT(bs->file, BLKDBG_L2_LOAD);
+        }
+
         ret = bdrv_pread(bs->file, offset, c->entries[i].table, s->cluster_size);
         if (ret < 0) {
             return ret;
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index c3ef550..76e7e07 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -67,7 +67,11 @@ int qcow2_grow_l1_table(BlockDriverState *bs, int min_size, bool exact_size)
         qemu_free(new_l1_table);
         return new_l1_table_offset;
     }
-    bdrv_flush(bs->file);
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        return ret;
+    }
 
     BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_WRITE_TABLE);
     for(i = 0; i < s->l1_size; i++)
@@ -98,63 +102,6 @@ int qcow2_grow_l1_table(BlockDriverState *bs, int min_size, bool exact_size)
     return ret;
 }
 
-void qcow2_l2_cache_reset(BlockDriverState *bs)
-{
-    BDRVQcowState *s = bs->opaque;
-
-    memset(s->l2_cache, 0, s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t));
-    memset(s->l2_cache_offsets, 0, L2_CACHE_SIZE * sizeof(uint64_t));
-    memset(s->l2_cache_counts, 0, L2_CACHE_SIZE * sizeof(uint32_t));
-}
-
-static inline int l2_cache_new_entry(BlockDriverState *bs)
-{
-    BDRVQcowState *s = bs->opaque;
-    uint32_t min_count;
-    int min_index, i;
-
-    /* find a new entry in the least used one */
-    min_index = 0;
-    min_count = 0xffffffff;
-    for(i = 0; i < L2_CACHE_SIZE; i++) {
-        if (s->l2_cache_counts[i] < min_count) {
-            min_count = s->l2_cache_counts[i];
-            min_index = i;
-        }
-    }
-    return min_index;
-}
-
-/*
- * seek_l2_table
- *
- * seek l2_offset in the l2_cache table
- * if not found, return NULL,
- * if found,
- *   increments the l2 cache hit count of the entry,
- *   if counter overflow, divide by two all counters
- *   return the pointer to the l2 cache entry
- *
- */
-
-static uint64_t *seek_l2_table(BDRVQcowState *s, uint64_t l2_offset)
-{
-    int i, j;
-
-    for(i = 0; i < L2_CACHE_SIZE; i++) {
-        if (l2_offset == s->l2_cache_offsets[i]) {
-            /* increment the hit count */
-            if (++s->l2_cache_counts[i] == 0xffffffff) {
-                for(j = 0; j < L2_CACHE_SIZE; j++) {
-                    s->l2_cache_counts[j] >>= 1;
-                }
-            }
-            return s->l2_cache + (i << s->l2_bits);
-        }
-    }
-    return NULL;
-}
-
 /*
  * l2_load
  *
@@ -169,33 +116,11 @@ static int l2_load(BlockDriverState *bs, uint64_t l2_offset,
     uint64_t **l2_table)
 {
     BDRVQcowState *s = bs->opaque;
-    int min_index;
     int ret;
 
-    /* seek if the table for the given offset is in the cache */
-
-    *l2_table = seek_l2_table(s, l2_offset);
-    if (*l2_table != NULL) {
-        return 0;
-    }
-
-    /* not found: load a new entry in the least used one */
-
-    min_index = l2_cache_new_entry(bs);
-    *l2_table = s->l2_cache + (min_index << s->l2_bits);
-
-    BLKDBG_EVENT(bs->file, BLKDBG_L2_LOAD);
-    ret = bdrv_pread(bs->file, l2_offset, *l2_table,
-        s->l2_size * sizeof(uint64_t));
-    if (ret < 0) {
-        qcow2_l2_cache_reset(bs);
-        return ret;
-    }
-
-    s->l2_cache_offsets[min_index] = l2_offset;
-    s->l2_cache_counts[min_index] = 1;
+    ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset, (void**) l2_table);
 
-    return 0;
+    return ret;
 }
 
 /*
@@ -238,7 +163,6 @@ static int write_l1_entry(BlockDriverState *bs, int l1_index)
 static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
 {
     BDRVQcowState *s = bs->opaque;
-    int min_index;
     uint64_t old_l2_offset;
     uint64_t *l2_table;
     int64_t l2_offset;
@@ -252,29 +176,48 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
     if (l2_offset < 0) {
         return l2_offset;
     }
-    bdrv_flush(bs->file);
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        goto fail;
+    }
 
     /* allocate a new entry in the l2 cache */
 
-    min_index = l2_cache_new_entry(bs);
-    l2_table = s->l2_cache + (min_index << s->l2_bits);
+    ret = qcow2_cache_get_empty(bs, s->l2_table_cache, l2_offset, (void**) table);
+    if (ret < 0) {
+        return ret;
+    }
+
+    l2_table = *table;
 
     if (old_l2_offset == 0) {
         /* if there was no old l2 table, clear the new table */
         memset(l2_table, 0, s->l2_size * sizeof(uint64_t));
     } else {
+        uint64_t* old_table;
+
         /* if there was an old l2 table, read it from the disk */
         BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_COW_READ);
-        ret = bdrv_pread(bs->file, old_l2_offset, l2_table,
-            s->l2_size * sizeof(uint64_t));
+        ret = qcow2_cache_get(bs, s->l2_table_cache, old_l2_offset,
+            (void**) &old_table);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        memcpy(l2_table, old_table, s->cluster_size);
+
+        ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &old_table);
         if (ret < 0) {
             goto fail;
         }
     }
+
     /* write the l2 table to the file */
     BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_WRITE);
-    ret = bdrv_pwrite_sync(bs->file, l2_offset, l2_table,
-        s->l2_size * sizeof(uint64_t));
+
+    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+    ret = qcow2_cache_flush(bs, s->l2_table_cache);
     if (ret < 0) {
         goto fail;
     }
@@ -286,17 +229,12 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
         goto fail;
     }
 
-    /* update the l2 cache entry */
-
-    s->l2_cache_offsets[min_index] = l2_offset;
-    s->l2_cache_counts[min_index] = 1;
-
     *table = l2_table;
     return 0;
 
 fail:
+    qcow2_cache_put(bs, s->l2_table_cache, (void**) table);
     s->l1_table[l1_index] = old_l2_offset;
-    qcow2_l2_cache_reset(bs);
     return ret;
 }
 
@@ -521,6 +459,8 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
                 &l2_table[l2_index], 0, QCOW_OFLAG_COPIED);
     }
 
+    qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+
    nb_available = (c * s->cluster_sectors);
 out:
     if (nb_available > nb_needed)
@@ -575,6 +515,7 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
             return ret;
         }
     } else {
+        /* FIXME Order */
         if (l2_offset)
             qcow2_free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t));
         ret = l2_allocate(bs, l1_index, &l2_table);
@@ -632,6 +573,7 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
 
     cluster_offset = qcow2_alloc_bytes(bs, compressed_size);
     if (cluster_offset < 0) {
+        qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
         return 0;
     }
 
@@ -646,38 +588,14 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
     /* compressed clusters never have the copied flag */
 
     BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE_COMPRESSED);
+    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
     l2_table[l2_index] = cpu_to_be64(cluster_offset);
-    if (bdrv_pwrite_sync(bs->file,
-                    l2_offset + l2_index * sizeof(uint64_t),
-                    l2_table + l2_index,
-                    sizeof(uint64_t)) < 0)
-        return 0;
-
-    return cluster_offset;
-}
-
-/*
- * Write L2 table updates to disk, writing whole sectors to avoid a
- * read-modify-write in bdrv_pwrite
- */
-#define L2_ENTRIES_PER_SECTOR (512 / 8)
-static int write_l2_entries(BlockDriverState *bs, uint64_t *l2_table,
-    uint64_t l2_offset, int l2_index, int num)
-{
-    int l2_start_index = l2_index & ~(L1_ENTRIES_PER_SECTOR - 1);
-    int start_offset = (8 * l2_index) & ~511;
-    int end_offset = (8 * (l2_index + num) + 511) & ~511;
-    size_t len = end_offset - start_offset;
-    int ret;
-
-    BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE);
-    ret = bdrv_pwrite(bs->file, l2_offset + start_offset,
-        &l2_table[l2_start_index], len);
+    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
     if (ret < 0) {
-        return ret;
+        return 0;
     }
 
-    return 0;
+    return cluster_offset;
 }
 
 int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
@@ -686,6 +604,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     int i, j = 0, l2_index, ret;
     uint64_t *old_cluster, start_sect, l2_offset, *l2_table;
     uint64_t cluster_offset = m->cluster_offset;
+    bool cow = false;
 
     if (m->nb_clusters == 0)
         return 0;
@@ -695,6 +614,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     /* copy content of unmodified sectors */
     start_sect = (m->offset & ~(s->cluster_size - 1)) >> 9;
     if (m->n_start) {
+        cow = true;
         ret = copy_sectors(bs, start_sect, cluster_offset, 0, m->n_start);
         if (ret < 0)
             goto err;
@@ -702,17 +622,30 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
 
     if (m->nb_available & (s->cluster_sectors - 1)) {
         uint64_t end = m->nb_available & ~(uint64_t)(s->cluster_sectors - 1);
+        cow = true;
         ret = copy_sectors(bs, start_sect + end, cluster_offset + (end << 9),
                 m->nb_available - end, s->cluster_sectors);
         if (ret < 0)
             goto err;
     }
 
-    /* update L2 table */
+    /*
+     * Update L2 table.
+     *
+     * Before we update the L2 table to actually point to the new cluster, we
+     * need to be sure that the refcounts have been increased and COW was
+     * handled.
+     */
+    if (cow) {
+        bdrv_flush(bs->file);
+    }
+
+    qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache);
     ret = get_cluster_table(bs, m->offset, &l2_table, &l2_offset, &l2_index);
     if (ret < 0) {
         goto err;
     }
+    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
 
     for (i = 0; i < m->nb_clusters; i++) {
         /* if two concurrent writes happen to the same unallocated cluster
@@ -728,16 +661,9 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
                     (i << s->cluster_bits)) | QCOW_OFLAG_COPIED);
      }
 
-    /*
-     * Before we update the L2 table to actually point to the new cluster, we
-     * need to be sure that the refcounts have been increased and COW was
-     * handled.
-     */
-    bdrv_flush(bs->file);
 
-    ret = write_l2_entries(bs, l2_table, l2_offset, l2_index, m->nb_clusters);
+    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
     if (ret < 0) {
-        qcow2_l2_cache_reset(bs);
         goto err;
     }
 
@@ -746,7 +672,6 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
      * Also flush bs->file to get the right order for L2 and refcount update.
      */
     if (j != 0) {
-        bdrv_flush(bs->file);
         for (i = 0; i < j; i++) {
             qcow2_free_any_clusters(bs,
                 be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED, 1);
@@ -868,7 +793,8 @@ int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
                 m->depends_on = old_alloc;
                 m->nb_clusters = 0;
                 *num = 0;
-                return 0;
+                ret = 0;
+                goto fail;
             }
         }
     }
@@ -884,7 +810,8 @@ int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
     cluster_offset = qcow2_alloc_clusters(bs, nb_clusters * s->cluster_size);
     if (cluster_offset < 0) {
         QLIST_REMOVE(m, next_in_flight);
-        return cluster_offset;
+        ret = cluster_offset;
+        goto fail;
     }
 
     /* save info needed for meta data update */
@@ -893,12 +820,21 @@ int qcow2_alloc_cluster_offset(BlockDriverState *bs, uint64_t offset,
     m->nb_clusters = nb_clusters;
 
 out:
+    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+    if (ret < 0) {
+        return ret;
+    }
+
     m->nb_available = MIN(nb_clusters << (s->cluster_bits - 9), n_end);
     m->cluster_offset = cluster_offset;
 
     *num = m->nb_available - n_start;
 
     return 0;
+
+fail:
+    qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+    return ret;
 }
 
 static int decompress_buffer(uint8_t *out_buf, int out_buf_size,
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index a10453c..e37e226 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -32,27 +32,6 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
                             int addend);
 
 
-static int cache_refcount_updates = 0;
-
-static int write_refcount_block(BlockDriverState *bs)
-{
-    BDRVQcowState *s = bs->opaque;
-    size_t size = s->cluster_size;
-
-    if (s->refcount_block_cache_offset == 0) {
-        return 0;
-    }
-
-    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_UPDATE);
-    if (bdrv_pwrite_sync(bs->file, s->refcount_block_cache_offset,
-            s->refcount_block_cache, size) < 0)
-    {
-        return -EIO;
-    }
-
-    return 0;
-}
-
 /*********************************************************/
 /* refcount handling */
 
@@ -61,7 +40,6 @@ int qcow2_refcount_init(BlockDriverState *bs)
     BDRVQcowState *s = bs->opaque;
     int ret, refcount_table_size2, i;
 
-    s->refcount_block_cache = qemu_malloc(s->cluster_size);
     refcount_table_size2 = s->refcount_table_size * sizeof(uint64_t);
     s->refcount_table = qemu_malloc(refcount_table_size2);
     if (s->refcount_table_size > 0) {
@@ -81,34 +59,22 @@ int qcow2_refcount_init(BlockDriverState *bs)
 void qcow2_refcount_close(BlockDriverState *bs)
 {
     BDRVQcowState *s = bs->opaque;
-    qemu_free(s->refcount_block_cache);
     qemu_free(s->refcount_table);
 }
 
 
 static int load_refcount_block(BlockDriverState *bs,
-                               int64_t refcount_block_offset)
+                               int64_t refcount_block_offset,
+                               void **refcount_block)
 {
     BDRVQcowState *s = bs->opaque;
     int ret;
 
-    if (cache_refcount_updates) {
-        ret = write_refcount_block(bs);
-        if (ret < 0) {
-            return ret;
-        }
-    }
-
     BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_LOAD);
-    ret = bdrv_pread(bs->file, refcount_block_offset, s->refcount_block_cache,
-                     s->cluster_size);
-    if (ret < 0) {
-        s->refcount_block_cache_offset = 0;
-        return ret;
-    }
+    ret = qcow2_cache_get(bs, s->refcount_block_cache, refcount_block_offset,
+        refcount_block);
 
-    s->refcount_block_cache_offset = refcount_block_offset;
-    return 0;
+    return ret;
 }
 
 /*
@@ -122,6 +88,8 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
     int refcount_table_index, block_index;
     int64_t refcount_block_offset;
     int ret;
+    uint16_t *refcount_block;
+    uint16_t refcount;
 
     refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
     if (refcount_table_index >= s->refcount_table_size)
@@ -129,16 +97,24 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
     refcount_block_offset = s->refcount_table[refcount_table_index];
     if (!refcount_block_offset)
         return 0;
-    if (refcount_block_offset != s->refcount_block_cache_offset) {
-        /* better than nothing: return allocated if read error */
-        ret = load_refcount_block(bs, refcount_block_offset);
-        if (ret < 0) {
-            return ret;
-        }
+
+    ret = qcow2_cache_get(bs, s->refcount_block_cache, refcount_block_offset,
+        (void**) &refcount_block);
+    if (ret < 0) {
+        return ret;
     }
+
     block_index = cluster_index &
         ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
-    return be16_to_cpu(s->refcount_block_cache[block_index]);
+    refcount = be16_to_cpu(refcount_block[block_index]);
+
+    ret = qcow2_cache_put(bs, s->refcount_block_cache,
+        (void**) &refcount_block);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return refcount;
 }
 
 /*
@@ -174,9 +150,10 @@ static int in_same_refcount_block(BDRVQcowState *s, uint64_t offset_a,
  * Loads a refcount block. If it doesn't exist yet, it is allocated first
  * (including growing the refcount table if needed).
  *
- * Returns the offset of the refcount block on success or -errno in error case
+ * Returns 0 on success or -errno in error case
  */
-static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
+static int alloc_refcount_block(BlockDriverState *bs,
+    int64_t cluster_index, uint16_t **refcount_block)
 {
     BDRVQcowState *s = bs->opaque;
     unsigned int refcount_table_index;
@@ -194,13 +171,8 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 
         /* If it's already there, we're done */
         if (refcount_block_offset) {
-            if (refcount_block_offset != s->refcount_block_cache_offset) {
-                ret = load_refcount_block(bs, refcount_block_offset);
-                if (ret < 0) {
-                    return ret;
-                }
-            }
-            return refcount_block_offset;
+             return load_refcount_block(bs, refcount_block_offset,
+                 (void**) refcount_block);
         }
     }
 
@@ -226,12 +198,10 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
      *   refcount block into the cache
      */
 
-    if (cache_refcount_updates) {
-        ret = write_refcount_block(bs);
-        if (ret < 0) {
-            return ret;
-        }
-    }
+    *refcount_block = NULL;
+
+    /* We write to the refcount table, so we might depend on L2 tables */
+    qcow2_cache_flush(bs, s->l2_table_cache);
 
     /* Allocate the refcount block itself and mark it as used */
     int64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
@@ -247,13 +217,18 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 
     if (in_same_refcount_block(s, new_block, cluster_index << s->cluster_bits)) {
         /* Zero the new refcount block before updating it */
-        memset(s->refcount_block_cache, 0, s->cluster_size);
-        s->refcount_block_cache_offset = new_block;
+        ret = qcow2_cache_get_empty(bs, s->refcount_block_cache, new_block,
+            (void**) refcount_block);
+        if (ret < 0) {
+            goto fail_block;
+        }
+
+        memset(*refcount_block, 0, s->cluster_size);
 
         /* The block describes itself, need to update the cache */
         int block_index = (new_block >> s->cluster_bits) &
             ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
-        s->refcount_block_cache[block_index] = cpu_to_be16(1);
+        (*refcount_block)[block_index] = cpu_to_be16(1);
     } else {
         /* Described somewhere else. This can recurse at most twice before we
          * arrive at a block that describes itself. */
@@ -266,14 +241,19 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 
         /* Initialize the new refcount block only after updating its refcount,
          * update_refcount uses the refcount cache itself */
-        memset(s->refcount_block_cache, 0, s->cluster_size);
-        s->refcount_block_cache_offset = new_block;
+        ret = qcow2_cache_get_empty(bs, s->refcount_block_cache, new_block,
+            (void**) refcount_block);
+        if (ret < 0) {
+            goto fail_block;
+        }
+
+        memset(*refcount_block, 0, s->cluster_size);
     }
 
     /* Now the new refcount block needs to be written to disk */
     BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_WRITE);
-    ret = bdrv_pwrite_sync(bs->file, new_block, s->refcount_block_cache,
-        s->cluster_size);
+    qcow2_cache_entry_mark_dirty(s->refcount_block_cache, *refcount_block);
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
     if (ret < 0) {
         goto fail_block;
     }
@@ -290,7 +270,12 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
         }
 
         s->refcount_table[refcount_table_index] = new_block;
-        return new_block;
+        return 0;
+    }
+
+    ret = qcow2_cache_put(bs, s->refcount_block_cache, (void**) refcount_block);
+    if (ret < 0) {
+        goto fail_block;
     }
 
     /*
@@ -410,9 +395,9 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
     qcow2_free_clusters(bs, old_table_offset, old_table_size * sizeof(uint64_t));
     s->free_cluster_index = old_free_cluster_index;
 
-    ret = load_refcount_block(bs, new_block);
+    ret = load_refcount_block(bs, new_block, (void**) refcount_block);
     if (ret < 0) {
-        goto fail_block;
+        return ret;
     }
 
     return new_block;
@@ -420,41 +405,10 @@ static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 fail_table:
     qemu_free(new_table);
 fail_block:
-    s->refcount_block_cache_offset = 0;
-    return ret;
-}
-
-#define REFCOUNTS_PER_SECTOR (512 >> REFCOUNT_SHIFT)
-static int write_refcount_block_entries(BlockDriverState *bs,
-    int64_t refcount_block_offset, int first_index, int last_index)
-{
-    BDRVQcowState *s = bs->opaque;
-    size_t size;
-    int ret;
-
-    if (cache_refcount_updates) {
-        return 0;
-    }
-
-    if (first_index < 0) {
-        return 0;
-    }
-
-    first_index &= ~(REFCOUNTS_PER_SECTOR - 1);
-    last_index = (last_index + REFCOUNTS_PER_SECTOR)
-        & ~(REFCOUNTS_PER_SECTOR - 1);
-
-    size = (last_index - first_index) << REFCOUNT_SHIFT;
-
-    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_UPDATE_PART);
-    ret = bdrv_pwrite(bs->file,
-        refcount_block_offset + (first_index << REFCOUNT_SHIFT),
-        &s->refcount_block_cache[first_index], size);
-    if (ret < 0) {
-        return ret;
+    if (*refcount_block != NULL) {
+        qcow2_cache_put(bs, s->refcount_block_cache, (void**) refcount_block);
     }
-
-    return 0;
+    return ret;
 }
 
 /* XXX: cache several refcount block clusters ? */
@@ -463,9 +417,8 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
 {
     BDRVQcowState *s = bs->opaque;
     int64_t start, last, cluster_offset;
-    int64_t refcount_block_offset = 0;
-    int64_t table_index = -1, old_table_index;
-    int first_index = -1, last_index = -1;
+    uint16_t *refcount_block = NULL;
+    int64_t old_table_index = -1;
     int ret;
 
 #ifdef DEBUG_ALLOC2
@@ -478,6 +431,11 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
         return 0;
     }
 
+    if (addend < 0) {
+        qcow2_cache_set_dependency(bs, s->refcount_block_cache,
+            s->l2_table_cache);
+    }
+
     start = offset & ~(s->cluster_size - 1);
     last = (offset + length - 1) & ~(s->cluster_size - 1);
     for(cluster_offset = start; cluster_offset <= last;
@@ -485,42 +443,33 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
     {
         int block_index, refcount;
         int64_t cluster_index = cluster_offset >> s->cluster_bits;
-        int64_t new_block;
+        int64_t table_index =
+            cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
 
-        /* Only write refcount block to disk when we are done with it */
-        old_table_index = table_index;
-        table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
-        if ((old_table_index >= 0) && (table_index != old_table_index)) {
+        /* Load the refcount block and allocate it if needed */
+        if (table_index != old_table_index) {
+            if (refcount_block) {
+                ret = qcow2_cache_put(bs, s->refcount_block_cache,
+                    (void**) &refcount_block);
+                if (ret < 0) {
+                    goto fail;
+                }
+            }
 
-            ret = write_refcount_block_entries(bs, refcount_block_offset,
-                first_index, last_index);
+            ret = alloc_refcount_block(bs, cluster_index, &refcount_block);
             if (ret < 0) {
-                return ret;
+                goto fail;
             }
-
-            first_index = -1;
-            last_index = -1;
         }
+        old_table_index = table_index;
 
-        /* Load the refcount block and allocate it if needed */
-        new_block = alloc_refcount_block(bs, cluster_index);
-        if (new_block < 0) {
-            ret = new_block;
-            goto fail;
-        }
-        refcount_block_offset = new_block;
+        qcow2_cache_entry_mark_dirty(s->refcount_block_cache, refcount_block);
 
         /* we can update the count and save it */
         block_index = cluster_index &
             ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
-        if (first_index == -1 || block_index < first_index) {
-            first_index = block_index;
-        }
-        if (block_index > last_index) {
-            last_index = block_index;
-        }
 
-        refcount = be16_to_cpu(s->refcount_block_cache[block_index]);
+        refcount = be16_to_cpu(refcount_block[block_index]);
         refcount += addend;
         if (refcount < 0 || refcount > 0xffff) {
             ret = -EINVAL;
@@ -529,17 +478,16 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
         if (refcount == 0 && cluster_index < s->free_cluster_index) {
             s->free_cluster_index = cluster_index;
         }
-        s->refcount_block_cache[block_index] = cpu_to_be16(refcount);
+        refcount_block[block_index] = cpu_to_be16(refcount);
     }
 
     ret = 0;
 fail:
-
     /* Write last changed block to disk */
-    if (refcount_block_offset != 0) {
+    if (refcount_block) {
         int wret;
-        wret = write_refcount_block_entries(bs, refcount_block_offset,
-            first_index, last_index);
+        wret = qcow2_cache_put(bs, s->refcount_block_cache,
+            (void**) &refcount_block);
         if (wret < 0) {
             return ret < 0 ? ret : wret;
         }
@@ -758,9 +706,7 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2, l1_allocated;
     int64_t old_offset, old_l2_offset;
     int l2_size, i, j, l1_modified, l2_modified, nb_csectors, refcount;
-
-    qcow2_l2_cache_reset(bs);
-    cache_refcount_updates = 1;
+    int ret;
 
     l2_table = NULL;
     l1_table = NULL;
@@ -784,7 +730,6 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     }
 
     l2_size = s->l2_size * sizeof(uint64_t);
-    l2_table = qemu_malloc(l2_size);
     l1_modified = 0;
     for(i = 0; i < l1_size; i++) {
         l2_offset = l1_table[i];
@@ -792,8 +737,13 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
             old_l2_offset = l2_offset;
             l2_offset &= ~QCOW_OFLAG_COPIED;
             l2_modified = 0;
-            if (bdrv_pread(bs->file, l2_offset, l2_table, l2_size) != l2_size)
+
+            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
+                (void**) &l2_table);
+            if (ret < 0) {
                 goto fail;
+            }
+
             for(j = 0; j < s->l2_size; j++) {
                 offset = be64_to_cpu(l2_table[j]);
                 if (offset != 0) {
@@ -833,17 +783,23 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
                         offset |= QCOW_OFLAG_COPIED;
                     }
                     if (offset != old_offset) {
+                        if (addend > 0) {
+                            qcow2_cache_set_dependency(bs, s->l2_table_cache,
+                                s->refcount_block_cache);
+                        }
                         l2_table[j] = cpu_to_be64(offset);
                         l2_modified = 1;
+                        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
                     }
                 }
             }
-            if (l2_modified) {
-                if (bdrv_pwrite_sync(bs->file,
-                                l2_offset, l2_table, l2_size) < 0)
-                    goto fail;
+
+            ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+            if (ret < 0) {
+                goto fail;
             }
 
+
             if (addend != 0) {
                 refcount = update_cluster_refcount(bs, l2_offset >> s->cluster_bits, addend);
             } else {
@@ -871,16 +827,14 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
     }
     if (l1_allocated)
         qemu_free(l1_table);
-    qemu_free(l2_table);
-    cache_refcount_updates = 0;
-    write_refcount_block(bs);
     return 0;
  fail:
+    if (l2_table) {
+        qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
+    }
+
     if (l1_allocated)
         qemu_free(l1_table);
-    qemu_free(l2_table);
-    cache_refcount_updates = 0;
-    write_refcount_block(bs);
     return -EIO;
 }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index b6b094c..49bf7b9 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -143,6 +143,7 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     int len, i, ret = 0;
     QCowHeader header;
     uint64_t ext_end;
+    bool writethrough;
 
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
@@ -217,8 +218,13 @@ static int qcow2_open(BlockDriverState *bs, int flags)
             be64_to_cpus(&s->l1_table[i]);
         }
     }
-    /* alloc L2 cache */
-    s->l2_cache = qemu_malloc(s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t));
+
+    /* alloc L2 table/refcount block cache */
+    writethrough = ((flags & BDRV_O_CACHE_MASK) == 0);
+    s->l2_table_cache = qcow2_cache_create(bs, L2_CACHE_SIZE, writethrough);
+    s->refcount_block_cache = qcow2_cache_create(bs, REFCOUNT_CACHE_SIZE,
+        writethrough);
+
     s->cluster_cache = qemu_malloc(s->cluster_size);
     /* one more sector for decompressed data alignment */
     s->cluster_data = qemu_malloc(QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size
@@ -270,7 +276,9 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     qcow2_free_snapshots(bs);
     qcow2_refcount_close(bs);
     qemu_free(s->l1_table);
-    qemu_free(s->l2_cache);
+    if (s->l2_table_cache) {
+        qcow2_cache_destroy(bs, s->l2_table_cache);
+    }
     qemu_free(s->cluster_cache);
     qemu_free(s->cluster_data);
     return ret;
@@ -719,7 +727,13 @@ static void qcow2_close(BlockDriverState *bs)
 {
     BDRVQcowState *s = bs->opaque;
     qemu_free(s->l1_table);
-    qemu_free(s->l2_cache);
+
+    qcow2_cache_flush(bs, s->l2_table_cache);
+    qcow2_cache_flush(bs, s->refcount_block_cache);
+
+    qcow2_cache_destroy(bs, s->l2_table_cache);
+    qcow2_cache_destroy(bs, s->refcount_block_cache);
+
     qemu_free(s->cluster_cache);
     qemu_free(s->cluster_data);
     qcow2_refcount_close(bs);
@@ -1179,6 +1193,19 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
 
 static int qcow2_flush(BlockDriverState *bs)
 {
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+
+    ret = qcow2_cache_flush(bs, s->l2_table_cache);
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        return ret;
+    }
+
     return bdrv_flush(bs->file);
 }
 
@@ -1186,6 +1213,19 @@ static BlockDriverAIOCB *qcow2_aio_flush(BlockDriverState *bs,
                                          BlockDriverCompletionFunc *cb,
                                          void *opaque)
 {
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+
+    ret = qcow2_cache_flush(bs, s->l2_table_cache);
+    if (ret < 0) {
+        return NULL;
+    }
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        return NULL;
+    }
+
     return bdrv_aio_flush(bs->file, cb, opaque);
 }
 
diff --git a/block/qcow2.h b/block/qcow2.h
index e5473e1..11cbce3 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -51,6 +51,9 @@
 
 #define L2_CACHE_SIZE 16
 
+/* Must be at least 4 to cover all cases of refcount table growth */
+#define REFCOUNT_CACHE_SIZE 4
+
 typedef struct QCowHeader {
     uint32_t magic;
     uint32_t version;
@@ -94,9 +97,10 @@ typedef struct BDRVQcowState {
     uint64_t cluster_offset_mask;
     uint64_t l1_table_offset;
     uint64_t *l1_table;
-    uint64_t *l2_cache;
-    uint64_t l2_cache_offsets[L2_CACHE_SIZE];
-    uint32_t l2_cache_counts[L2_CACHE_SIZE];
+
+    Qcow2Cache* l2_table_cache;
+    Qcow2Cache* refcount_block_cache;
+
     uint8_t *cluster_cache;
     uint8_t *cluster_data;
     uint64_t cluster_cache_offset;
@@ -105,8 +109,6 @@ typedef struct BDRVQcowState {
     uint64_t *refcount_table;
     uint64_t refcount_table_offset;
     uint32_t refcount_table_size;
-    uint64_t refcount_block_cache_offset;
-    uint16_t *refcount_block_cache;
     int64_t free_cluster_index;
     int64_t free_byte_offset;
 
-- 
1.7.2.3

[Qemu-devel] [PATCH v5 3/3] qcow2: Batch flushes for COW

[Kevin Wolf]

qcow2 calls bdrv_flush() after performing COW in order to ensure that the
L2 table change is never written before the copy is safe on disk. Now that the
L2 table is cached, we can wait with flushing until we write out the next L2
table.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cache.c   |   20 +++++++++++++++++---
 block/qcow2-cluster.c |    2 +-
 block/qcow2.h         |    1 +
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 5f1740b..8f2955b 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -38,6 +38,7 @@ struct Qcow2Cache {
     int                     size;
     Qcow2CachedTable*       entries;
     struct Qcow2Cache*      depends;
+    bool                    depends_on_flush;
     bool                    writethrough;
 };
 
@@ -85,13 +86,15 @@ static int qcow2_cache_flush_dependency(BlockDriverState *bs, Qcow2Cache *c)
     }
 
     c->depends = NULL;
+    c->depends_on_flush = false;
+
     return 0;
 }
 
 static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
 {
     BDRVQcowState *s = bs->opaque;
-    int ret;
+    int ret = 0;
 
     if (!c->entries[i].dirty || !c->entries[i].offset) {
         return 0;
@@ -99,11 +102,17 @@ static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
 
     if (c->depends) {
         ret = qcow2_cache_flush_dependency(bs, c);
-        if (ret < 0) {
-            return ret;
+    } else if (c->depends_on_flush) {
+        ret = bdrv_flush(bs->file);
+        if (ret >= 0) {
+            c->depends_on_flush = false;
         }
     }
 
+    if (ret < 0) {
+        return ret;
+    }
+
     if (c == s->refcount_block_cache) {
         BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_UPDATE_PART);
     } else if (c == s->l2_table_cache) {
@@ -167,6 +176,11 @@ int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     return 0;
 }
 
+void qcow2_cache_depends_on_flush(Qcow2Cache *c)
+{
+    c->depends_on_flush = true;
+}
+
 static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
 {
     int i;
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 76e7e07..1c2003a 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -637,7 +637,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
      * handled.
      */
     if (cow) {
-        bdrv_flush(bs->file);
+        qcow2_cache_depends_on_flush(s->l2_table_cache);
     }
 
     qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache);
diff --git a/block/qcow2.h b/block/qcow2.h
index 11cbce3..6d80120 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -229,6 +229,7 @@ void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
 int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
 int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency);
+void qcow2_cache_depends_on_flush(Qcow2Cache *c);
 
 int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
     void **table);
-- 
1.7.2.3

Re: [Qemu-devel] [PATCH v5 0/3] qcow2 metadata cache

[TeLeMan]

These patches break booting Windows.

Re: [Qemu-devel] [PATCH v5 0/3] qcow2 metadata cache

[Kevin Wolf]

Am 27.01.2011 08:52, schrieb TeLeMan:
> These patches break booting Windows.

Can you be a bit more specific? It certainly doesn't break my Windows
images. Just to be sure that I didn't miss a last-minute regression, I
just installed another Win 7 VM and it works just fine.

Which Windows version do you use? What's your command line? Do you get
block error events on a QMP monitor?

Kevin

Re: [Qemu-devel] [PATCH v5 0/3] qcow2 metadata cache

[TeLeMan]

Host: Win7
Guest: WinXP

Reproduce:
qemu-img create -f qcow2 winxp.qcow2 80G
qemu -L pc-bios -m 256 -hda winxp.qcow2 -cdrom WXPVOL_EN.iso -boot d

Error:
An unexpected error (3073) occurred at
line 5137 in d:\xp\client\base\boot\setup\setup.c

I tried adding "-snapshot" into the command line, it worked.