Re: [Qemu-devel] [PATCH v13 00/10] Block replication for continuous checkpoints

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wen Congyang (wency@cn.fujitsu.com) wrote:
> On 01/29/2016 06:07 PM, Dr. David Alan Gilbert wrote:
> > * 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
> 
> Note, the count is 1, not 2. Writing to children.0 is in flight. If writing to children.0 successes,
> the guest doesn't know this error.
> >> 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?
> 
> We will write to two children, and expect that writing to children.0 will success. If so,
> the guest doesn't know this error. You just get the I/O error event.

I think children.0 is the disk, and that should be OK - so only the children.1/replication should
be failing - so in that case why do I see the error?
The 'node0' in the backtrace above is the name of the replication, so it does look like the error
is coming from the replication.

> > 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 
> 
> Hmm, you should not remove the child before failover. I will check it how to avoid it in the codes.

 But you said 'If secondary host is down, you should remove quorum's children.1' - is that not
what you meant?

> > 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?
> 
> Yes, but in the newest version, quorum doesn't know the block replication, and I think
> we shoud add an reference to the bs when starting block replication.

Do you have a new version ready to test?  I'm interested to try it (and also interested
to try the latest version of the colo-proxy)

Dave

> Thanks
> Wen Congyang
> 
> > (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
> > 
> > 
> > .
> > 
> 
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK