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Re: [Qemu-devel] Qemu and Changed Block Tracking

From: Peter Lieven
Subject: Re: [Qemu-devel] Qemu and Changed Block Tracking
Date: Sun, 26 Feb 2017 21:41:17 +0100

> Am 24.02.2017 um 22:44 schrieb Eric Blake <address@hidden>:
> On 02/24/2017 03:31 PM, John Snow wrote:
>>> But the Backup Server could instead connect to the NAS directly avoiding
>>> load on the frontent LAN
>>> and the Qemu Node.
>> In a live backup I don't see how you will be removing QEMU from the data
>> transfer loop. QEMU is the only process that knows what the correct view
>> of the image is, and needs to facilitate.
>> It's not safe to copy the blocks directly without QEMU's mediation.
> Although we may already have enough tools in place to help achieve that:
> create a temporary qcow2 wrapper around the primary image via external
> snapshot, so that the primary image is now read-only in qemu; then use
> whatever block-status mechanism (whether the NBD block status extension,
> or directly reading from a persistent bitmap) to facilitate whatever
> more efficient offline transfer of just the relevant portions of that
> main file, then live block-commit to get qemu to start writing to the
> file again.
> In other words, any time your algorithm wants to cause an I/O freeze to
> a particular file, the solution is to add a qcow2 external snapshot
> followed by a live commit.
> So tweaking the proposal a few mails ago:
> fsfreeze (optional)
> create qcow2 snapshot wrapper as a write lock (via QMP)
> fsthaw - now with no risk of violating guest timing constraints
> dirtymap = find all blocks that are dirty since last backup (via named
> bitmap/NBD block status)
> foreach block in dirtymap {
>               copy to backup via external software
> }
> live commit image (via QMP)
> The window where guest I/O is frozen is small (the freeze/snapshot
> create/thaw steps can be done in less than a second), while the window
> where you are extracting incremental backup data is longer (during that
> time, guest I/O is happening into a wrapper qcow2 file).

The live-snapshot/live-commit stuff could indeed help in my scenario. If I 
understand correctly this is
something that already works today, correct? If I have taken a live-snapshot, 
is live-migration and
stop/start of the VM still possible? What about live-migration and start/stop 
during live-commit?
I don’t talk about the dirty bitmap tracking I understand that persistence and 
live-migration support
is still in the works, I’m just interested in the snapshot/commit part.


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