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Re: [Qemu-devel] [PATCH v13 00/17] block: incremental backup series

From: John Snow
Subject: Re: [Qemu-devel] [PATCH v13 00/17] block: incremental backup series
Date: Mon, 23 Feb 2015 14:16:45 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 02/23/2015 12:52 PM, Stefan Hajnoczi wrote:
On Fri, Feb 20, 2015 at 12:20:53PM -0500, John Snow wrote:

Eric: I've added you on CC for a libvirt perspective on the APIs that
this series introduces.  We're discussing how incremental backup
failures should be handled when there are multiple drives attached to
the guest.  Please see towards the bottom of this email.

On 02/20/2015 06:09 AM, Stefan Hajnoczi wrote:
On Fri, Feb 13, 2015 at 05:08:41PM -0500, John Snow wrote:
This series requires: [PATCH v3] blkdebug: fix "once" rule

Welcome to the "incremental backup" newsletter, where we discuss
exciting developments in non-redundant backup technology.
This issue is called the "Max Reitz Fixup" issue.

This patchset enables the in-memory part of the incremental backup
feature. There are two series on the mailing list now by Vladimir
Sementsov-Ogievskiy that enable the migration and persistence of
dirty bitmaps.

This series and most patches were originally by Fam Zheng.

Please add docs/incremental-backup.txt explaining how the commands are
intended to be used to perform backups.  The QAPI documentation is
sparse and QMP clients would probably need to read the code to
understand how these commands work.

OK. I wrote a markdown formatted file that explains it all pretty well;
should I check it in as-is, or should I convert it to some other format?


Thanks, that is helpful.  The multi-disk use case is most complex,
please include it.

Please submit to docs/ as plain text.

I'm not sure I understand the need for all the commands: add, remove,
enable, disable, clear.  Why are these commands necessary?

add/remove are self explanatory.

clear allows you to re-sync a bitmap to a full backup after you have already
been running for some time. See the readme for the usage case. Yes, you
*COULD* delete and re-create the bitmap with add/remove, but why?
Clear is nice, I stand by it.

enable/disable: Not necessarily useful for the simple case at this exact
moment; they can be used to track writes during a period of time if desired.
You could use them with transactions to record activity through different
periods of time. They were originally used for what the "frozen" case covers
now, but I left them in.

I could stand to part with them if you think they detract more than help.
They seemed like useful primitives. My default action will be to leave them
in, still.

I think just add and remove should be enough:

Initial full backup
Use transaction to add bitmaps to all drives atomically (i.e.
point-in-time snapshot across all drives) and launch drive-backup (full)
on all drives.

In your patch the bitmap starts disabled.  In my example bitmaps are
always enabled unless they have a successor.

No they don't:

     bitmap->disabled = false;

Great, I missed that.  In that case enable/disable are truly optional.
They can be added in the future without needing to change the 'add'
command's default.

I will remove the interface, but the status will remain. Vladimir uses the read-only mode for migrations.

All code qemu.git requires code review, testing, maintenance, and in the
case of QMP, a commitment to keep the command forever.  Please drop
enable/disable for now.

Incremental backup
Use transaction to launch drive-backup (bitmap mode) on all drives.

If backup completes successfully we'll be able to run the same command
again for the next incremental backup.

If backup fails I see a problem when taking consistent incremental
backups across multiple drives:

Imagine 2 drives (A and B).  Transaction is used to launch drive-backup
on both with their respective dirty bitmaps.  drive-backup A fails and
merges successor dirty bits so nothing is lost, but what do we do with
drive-backup B?

drive-backup B could be in-progress or it could have completed before
drive-backup A failed.  Now we're in trouble because we need to take
consistent snapshots across all drives but we've thrown away the dirty
bitmap for drive B!

Just re-run the transaction. If one failed but one succeeded, just run it
again. The one that succeeded prior will now have a trivial incremental
backup, and the one that failed will have a new valid incremental backup.
The two new incrementals will be point in time consistent.


[full_a] <-- [incremental_a_0: FAILED]
[full_b] <-- [incremental_b_0: SUCCESS]

then re-run:

[full_a] <------------------------ [incremental_a_1]
[full_b] <-- [incremental_b_0] <-- [incremental_b_1]

If the extra image in the chain for the one that succeeded is problematic,
you can use other tools to consolidate them later.

Each client implementation needs to duplicate the logic for partial
backups.  There can be arbitrary levels of partial backups since failure
could occur twice, three times, etc in a row.  We're forcing all clients
to deal with failure themselves or to fall back to full backup on

The "client" may be a backup application that wants QEMU to send data
directly to a NBD server.  In order for that to work, the backup
application also needs to keep state or fall back to full backup on any

In other words, making the API require a stateful client is ugly.

Every interface is beautiful in its own special way.

Either way, how do you propose getting a consistent snapshot across multiple
drives after a failure? The only recovery option is to just create a new
snapshot *later*, you can never go back to what was, just like you can't for
single drives.

This is the same problem as for a single drive.  There the solution is
to merge the old and new bitmaps, and the client has to take a later
snapshot like you explained.

That policy is fine and should apply to multiple drives too.  Take a new
snapshot if there was a failure.

OK; I will need to cook something up to allow us to run another action on all the drive backups after they all complete...

I am not convinced there is a problem. Since we don't want filename
management (&c) as part of this solution inside of QEMU, there is nothing
left to do except within libvirt.

No filenames are involved.  Just don't throw away the frozen dirty
bitmap until the entire group of backup operations completes.  It's
equivalent to what we do in the single drive case.

Stopping incremental backup
Use transaction to remove bitmaps on all drives.  This case is easy.

Finally, what about bdrv_truncate()?  When the disk size changes the
dirty bitmaps keep the old size.  Seems likely to cause problems.  Have
you thought about this case?

Only just recently when reviewing Vladimir's bitmap persistence patches,

This patch series needs to address the issue since the QMP API to create
bitmaps is exposed here.

The drive cannot be resized during the backup operation.  Luckily we're
already protected against that by the op blocker API.

The question is what happens to existing bitmaps (enabled, disabled, or
frozen) when there is no backup blockjob running and the drive is
resized.  I think it makes sense to resize the bitmaps along with the


It should not be possible to resize a frozen drive, since they only freeze during an operation.

I agree that read-only bitmaps should just go ahead and perform the resize, however.

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