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Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring |
Date: |
Tue, 19 Sep 2017 10:44:16 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
On Mon, Sep 18, 2017 at 06:26:51PM +0200, Max Reitz wrote:
> On 2017-09-18 12:06, Stefan Hajnoczi wrote:
> > On Sat, Sep 16, 2017 at 03:58:01PM +0200, Max Reitz wrote:
> >> On 2017-09-14 17:57, Stefan Hajnoczi wrote:
> >>> On Wed, Sep 13, 2017 at 08:19:07PM +0200, Max Reitz wrote:
> >>>> This patch implements active synchronous mirroring. In active mode, the
> >>>> passive mechanism will still be in place and is used to copy all
> >>>> initially dirty clusters off the source disk; but every write request
> >>>> will write data both to the source and the target disk, so the source
> >>>> cannot be dirtied faster than data is mirrored to the target. Also,
> >>>> once the block job has converged (BLOCK_JOB_READY sent), source and
> >>>> target are guaranteed to stay in sync (unless an error occurs).
> >>>>
> >>>> Optionally, dirty data can be copied to the target disk on read
> >>>> operations, too.
> >>>>
> >>>> Active mode is completely optional and currently disabled at runtime. A
> >>>> later patch will add a way for users to enable it.
> >>>>
> >>>> Signed-off-by: Max Reitz <address@hidden>
> >>>> ---
> >>>> qapi/block-core.json | 23 +++++++
> >>>> block/mirror.c | 187
> >>>> +++++++++++++++++++++++++++++++++++++++++++++++++--
> >>>> 2 files changed, 205 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/qapi/block-core.json b/qapi/block-core.json
> >>>> index bb11815608..e072cfa67c 100644
> >>>> --- a/qapi/block-core.json
> >>>> +++ b/qapi/block-core.json
> >>>> @@ -938,6 +938,29 @@
> >>>> 'data': ['top', 'full', 'none', 'incremental'] }
> >>>>
> >>>> ##
> >>>> +# @MirrorCopyMode:
> >>>> +#
> >>>> +# An enumeration whose values tell the mirror block job when to
> >>>> +# trigger writes to the target.
> >>>> +#
> >>>> +# @passive: copy data in background only.
> >>>> +#
> >>>> +# @active-write: when data is written to the source, write it
> >>>> +# (synchronously) to the target as well. In addition,
> >>>> +# data is copied in background just like in @passive
> >>>> +# mode.
> >>>> +#
> >>>> +# @active-read-write: write data to the target (synchronously) both
> >>>> +# when it is read from and written to the source.
> >>>> +# In addition, data is copied in background just
> >>>> +# like in @passive mode.
> >>>
> >>> I'm not sure the terms "active"/"passive" are helpful. "Active commit"
> >>> means committing the top-most BDS while the guest is accessing it. The
> >>> "passive" mirror block still works on the top-most BDS while the guest
> >>> is accessing it.
> >>>
> >>> Calling it "asynchronous" and "synchronous" is clearer to me. It's also
> >>> the terminology used in disk replication (e.g. DRBD).
> >>
> >> I'd be OK with that, too, but I think I remember that in the past at
> >> least Kevin made a clear distinction between active/passive and
> >> sync/async when it comes to mirroring.
> >>
> >>> Ideally the user wouldn't have to worry about async vs sync because QEMU
> >>> would switch modes as appropriate in order to converge. That way
> >>> libvirt also doesn't have to worry about this.
> >>
> >> So here you mean async/sync in the way I meant it, i.e., whether the
> >> mirror operations themselves are async/sync?
> >
> > The meaning I had in mind is:
> >
> > Sync mirroring means a guest write waits until the target write
> > completes.
>
> I.e. active-sync, ...
>
> > Async mirroring means guest writes completes independently of target
> > writes.
>
> ... i.e. passive or active-async in the future.
>
> So you really want qemu to decide whether to use active or passive mode
> depending on what's enough to let the block job converge and not
> introduce any switch for the user?
>
> I'm not sure whether I like this too much, mostly because "libvirt
> doesn't have to worry" doesn't feel quite right to me. If we don't make
> libvirt worry about this, then qemu has to worry. I'm not sure whether
> that's any better.
>
> I think this really does get into policy territory. Just switching to
> active mode the instant target writes are slower than source writes may
> not be what the user wants: Maybe it's OK for a short duration because
> they don't care about hard convergence too much. Maybe they want to
> switch to active mode already when "only" twice as much is written to
> the target as to the source.
>
> I think this is a decision the management layer (or the user) has to make.
Eric: Does libvirt want to be involved with converging the mirror job
(i.e. if the guest is writing to disk faster than QEMU can copy data to
the target)?
Stefan
- [Qemu-block] [PATCH 10/18] block/mirror: Make source the file child, (continued)
- [Qemu-block] [PATCH 10/18] block/mirror: Make source the file child, Max Reitz, 2017/09/13
- [Qemu-block] [PATCH 11/18] hbitmap: Add @advance param to hbitmap_iter_next(), Max Reitz, 2017/09/13
- [Qemu-block] [PATCH 12/18] block/dirty-bitmap: Add bdrv_dirty_iter_next_area, Max Reitz, 2017/09/13
- [Qemu-block] [PATCH 13/18] block/mirror: Keep write perm for pending writes, Max Reitz, 2017/09/13
- [Qemu-block] [PATCH 14/18] block/mirror: Distinguish active from passive ops, Max Reitz, 2017/09/13
- [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring, Max Reitz, 2017/09/13
- Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring, Stefan Hajnoczi, 2017/09/14
- Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring, Max Reitz, 2017/09/16
- Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring, Stefan Hajnoczi, 2017/09/18
- Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring, Max Reitz, 2017/09/18
- Re: [Qemu-block] [PATCH 15/18] block/mirror: Add active mirroring,
Stefan Hajnoczi <=
- Re: [Qemu-block] [Qemu-devel] [PATCH 15/18] block/mirror: Add active mirroring, Daniel P. Berrange, 2017/09/19
- Re: [Qemu-block] [Qemu-devel] [PATCH 15/18] block/mirror: Add active mirroring, Stefan Hajnoczi, 2017/09/20
[Qemu-block] [PATCH 16/18] block/mirror: Add copy mode QAPI interface, Max Reitz, 2017/09/13
[Qemu-block] [PATCH 17/18] qemu-io: Add background write, Max Reitz, 2017/09/13
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Fam Zheng, 2017/09/18
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Max Reitz, 2017/09/18
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Fam Zheng, 2017/09/19
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Max Reitz, 2017/09/21
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Fam Zheng, 2017/09/21
- Re: [Qemu-block] [PATCH 17/18] qemu-io: Add background write, Max Reitz, 2017/09/21