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Date: Fri, 29 Jan 2016 10:07:46 +0000
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Message-ID: <20160129100745.GA2410@work-vm>
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	<20160127110345.GB2394@work-vm> <56AB0C1F.90008@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] [PATCH v13 00/10] Block replication for continuous
	checkpoints
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To: Wen Congyang <wency@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Changlong Xie <xiecl.fnst@cn.fujitsu.com>, Fam Zheng <famz@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>, qemu block <qemu-block@nongnu.org>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, qemu devel <qemu-devel@nongnu.org>, "Michael R. Hines" <mrhines@linux.vnet.ibm.com>, Max Reitz <mreitz@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

* Wen Congyang (wency@cn.fujitsu.com) wrote:
> On 01/27/2016 07:03 PM, Dr. David Alan Gilbert wrote:
> > Hi,
> >   I've got a block error if I kill the secondary.
> > 
> > Start both primary & secondary
> > kill -9 secondary qemu
> > x_colo_lost_heartbeat on primary
> > 
> > The guest sees a block error and the ext4 root switches to read-only.
> > 
> > I gdb'd the primary with a breakpoint on quorum_report_bad; see
> > backtrace below.
> > (This is based on colo-v2.4-periodic-mode of the framework
> > code with the block and network proxy merged in; so it could be my
> > merging but I don't think so ?)
> > 
> > 
> > (gdb) where
> > #0  quorum_report_bad (node_name=0x7f2946a0892c "node0", ret=-5, acb=0x7f2946cb3910, acb=0x7f2946cb3910)
> >     at /root/colo/jan-2016/qemu/block/quorum.c:222
> > #1  0x00007f2943b23058 in quorum_aio_cb (opaque=<optimized out>, ret=<optimized out>)
> >     at /root/colo/jan-2016/qemu/block/quorum.c:315
> > #2  0x00007f2943b311be in bdrv_co_complete (acb=0x7f2946cb3f60) at /root/colo/jan-2016/qemu/block/io.c:2122
> > #3  0x00007f2943ae777d in aio_bh_call (bh=<optimized out>) at /root/colo/jan-2016/qemu/async.c:64
> > #4  aio_bh_poll (ctx=ctx@entry=0x7f2945b771d0) at /root/colo/jan-2016/qemu/async.c:92
> > #5  0x00007f2943af5090 in aio_dispatch (ctx=0x7f2945b771d0) at /root/colo/jan-2016/qemu/aio-posix.c:305
> > #6  0x00007f2943ae756e in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, 
> >     user_data=<optimized out>) at /root/colo/jan-2016/qemu/async.c:231
> > #7  0x00007f293b84a79a in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
> > #8  0x00007f2943af3a00 in glib_pollfds_poll () at /root/colo/jan-2016/qemu/main-loop.c:211
> > #9  os_host_main_loop_wait (timeout=<optimized out>) at /root/colo/jan-2016/qemu/main-loop.c:256
> > #10 main_loop_wait (nonblocking=<optimized out>) at /root/colo/jan-2016/qemu/main-loop.c:504
> > #11 0x00007f29438529ee in main_loop () at /root/colo/jan-2016/qemu/vl.c:1945
> > #12 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at /root/colo/jan-2016/qemu/vl.c:4707
> > 
> > (gdb) p s->num_children
> > $1 = 2
> > (gdb) p acb->success_count
> > $2 = 0
> > (gdb) p acb->is_read
> > $5 = false
> 
> Sorry for the late reply.

No problem.

> What it the value of acb->count?

(gdb) p acb->count
$1 = 1

> If secondary host is down, you should remove quorum's children.1. Otherwise, you will get
> I/O error event.

Is that safe?  If the secondary fails, do you always have time to issue the command to
remove the children.1  before the guest sees the error?

Anyway, I tried removing children.1 but it segfaults now, I guess the replication is unhappy:

(qemu) x_block_change colo-disk0 -d children.1
(qemu) x_colo_lost_heartbeat 

12973 Segmentation fault      (core dumped) ./try/x86_64-softmmu/qemu-system-x86_64 -enable-kvm $console_param -S -boot c -m 4080 -smp 4 -machine pc-i440fx-2.5,accel=kvm -name debug-threads=on -trace events=trace-file -device virtio-rng-pci $block_param $net_param

#0  0x00007f0a398a864c in bdrv_stop_replication (bs=0x7f0a3b0a8430, failover=true, errp=0x7fff6a5c3420)
    at /root/colo/jan-2016/qemu/block.c:4426

(gdb) p drv
$1 = (BlockDriver *) 0x5d2a

  it looks like the whole of bs is bogus.

#1  0x00007f0a398d87f6 in quorum_stop_replication (bs=<optimized out>, failover=<optimized out>, 
    errp=<optimized out>) at /root/colo/jan-2016/qemu/block/quorum.c:1213

(gdb) p s->replication_index
$3 = 1

I guess quorum_del_child needs to stop replication before it removes the child?
(although it would have to be careful not to block on the dead nbd).

#2  0x00007f0a398a8901 in bdrv_stop_replication_all (failover=failover@entry=true, errp=errp@entry=0x7fff6a5c3478)
    at /root/colo/jan-2016/qemu/block.c:4504
#3  0x00007f0a3984b0af in primary_vm_do_failover () at /root/colo/jan-2016/qemu/migration/colo.c:144
#4  colo_do_failover (s=<optimized out>) at /root/colo/jan-2016/qemu/migration/colo.c:162
#5  0x00007f0a3989d7fd in aio_bh_call (bh=<optimized out>) at /root/colo/jan-2016/qemu/async.c:64
#6  aio_bh_poll (ctx=ctx@entry=0x7f0a3a6c21d0) at /root/colo/jan-2016/qemu/async.c:92
#7  0x00007f0a398ab110 in aio_dispatch (ctx=0x7f0a3a6c21d0) at /root/colo/jan-2016/qemu/aio-posix.c:305
#8  0x00007f0a3989d5ee in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, 
    user_data=<optimized out>) at /root/colo/jan-2016/qemu/async.c:231
#9  0x00007f0a3160079a in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
#10 0x00007f0a398a9a80 in glib_pollfds_poll () at /root/colo/jan-2016/qemu/main-loop.c:211
#11 os_host_main_loop_wait (timeout=<optimized out>) at /root/colo/jan-2016/qemu/main-loop.c:256
#12 main_loop_wait (nonblocking=<optimized out>) at /root/colo/jan-2016/qemu/main-loop.c:504
#13 0x00007f0a396089ee in main_loop () at /root/colo/jan-2016/qemu/vl.c:1945
#14 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at /root/colo/jan-2016/qemu/vl.c:4707

Dave

> Thanks
> Wen Congyang
> 
> > 
> > (qemu) info block
> > colo-disk0 (#block080): json:{"children": [{"driver": "raw", "file": {"driver": "file", "filename": "/root/colo/bugzilla.raw"}}, {"driver": "replication", "mode": "primary", "file": {"port": "8889", "host": "ibpair", "driver": "nbd", "export": "colo-disk0"}}], "driver": "quorum", "blkverify": false, "rewrite-corrupted": false, "vote-threshold": 1} (quorum)
> >     Cache mode:       writeback, direct
> > 
> > Dave
> > 
> > * Changlong Xie (xiecl.fnst@cn.fujitsu.com) wrote:
> >> Block replication is a very important feature which is used for
> >> continuous checkpoints(for example: COLO).
> >>
> >> You can get the detailed information about block replication from here:
> >> http://wiki.qemu.org/Features/BlockReplication
> >>
> >> Usage:
> >> Please refer to docs/block-replication.txt
> >>
> >> This patch series is based on the following patch series:
> >> 1. http://lists.nongnu.org/archive/html/qemu-devel/2015-12/msg04570.html
> >>
> >> You can get the patch here:
> >> https://github.com/Pating/qemu/tree/changlox/block-replication-v13
> >>
> >> You can get the patch with framework here:
> >> https://github.com/Pating/qemu/tree/changlox/colo_framework_v12
> >>
> >> TODO:
> >> 1. Continuous block replication. It will be started after basic functions
> >>    are accepted.
> >>
> >> Changs Log:
> >> V13:
> >> 1. Rebase to the newest codes
> >> 2. Remove redundant marcos and semicolon in replication.c 
> >> 3. Fix typos in block-replication.txt
> >> V12:
> >> 1. Rebase to the newest codes
> >> 2. Use backing reference to replcace 'allow-write-backing-file'
> >> V11:
> >> 1. Reopen the backing file when starting blcok replication if it is not
> >>    opened in R/W mode
> >> 2. Unblock BLOCK_OP_TYPE_BACKUP_SOURCE and BLOCK_OP_TYPE_BACKUP_TARGET
> >>    when opening backing file
> >> 3. Block the top BDS so there is only one block job for the top BDS and
> >>    its backing chain.
> >> V10:
> >> 1. Use blockdev-remove-medium and blockdev-insert-medium to replace backing
> >>    reference.
> >> 2. Address the comments from Eric Blake
> >> V9:
> >> 1. Update the error messages
> >> 2. Rebase to the newest qemu
> >> 3. Split child add/delete support. These patches are sent in another patchset.
> >> V8:
> >> 1. Address Alberto Garcia's comments
> >> V7:
> >> 1. Implement adding/removing quorum child. Remove the option non-connect.
> >> 2. Simplify the backing refrence option according to Stefan Hajnoczi's suggestion
> >> V6:
> >> 1. Rebase to the newest qemu.
> >> V5:
> >> 1. Address the comments from Gong Lei
> >> 2. Speed the failover up. The secondary vm can take over very quickly even
> >>    if there are too many I/O requests.
> >> V4:
> >> 1. Introduce a new driver replication to avoid touch nbd and qcow2.
> >> V3:
> >> 1: use error_setg() instead of error_set()
> >> 2. Add a new block job API
> >> 3. Active disk, hidden disk and nbd target uses the same AioContext
> >> 4. Add a testcase to test new hbitmap API
> >> V2:
> >> 1. Redesign the secondary qemu(use image-fleecing)
> >> 2. Use Error objects to return error message
> >> 3. Address the comments from Max Reitz and Eric Blake
> >>
> >> Wen Congyang (10):
> >>   unblock backup operations in backing file
> >>   Store parent BDS in BdrvChild
> >>   Backup: clear all bitmap when doing block checkpoint
> >>   Allow creating backup jobs when opening BDS
> >>   docs: block replication's description
> >>   Add new block driver interfaces to control block replication
> >>   quorum: implement block driver interfaces for block replication
> >>   Implement new driver for block replication
> >>   support replication driver in blockdev-add
> >>   Add a new API to start/stop replication, do checkpoint to all BDSes
> >>
> >>  block.c                    | 145 ++++++++++++
> >>  block/Makefile.objs        |   3 +-
> >>  block/backup.c             |  14 ++
> >>  block/quorum.c             |  78 +++++++
> >>  block/replication.c        | 545 +++++++++++++++++++++++++++++++++++++++++++++
> >>  blockjob.c                 |  11 +
> >>  docs/block-replication.txt | 227 +++++++++++++++++++
> >>  include/block/block.h      |   9 +
> >>  include/block/block_int.h  |  15 ++
> >>  include/block/blockjob.h   |  12 +
> >>  qapi/block-core.json       |  33 ++-
> >>  11 files changed, 1089 insertions(+), 3 deletions(-)
> >>  create mode 100644 block/replication.c
> >>  create mode 100644 docs/block-replication.txt
> >>
> >> -- 
> >> 1.9.3
> >>
> >>
> >>
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> > 
> > .
> > 
> 
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK