03.04.2018 15:01, Nikolay Shirokovskiy wrote:
Hi, all.
This is another RFC on pull backup API. This API provides means to read domain
disks in a snapshotted state so that client can back them up as well as means
to write domain disks to revert them to backed up state. The previous version
of RFC is [1]. I'll also describe the API implementation details to shed light
on misc qemu dirty bitmap commands usage.


[...]

3. Checkpoints (the most interesting part)

First a few facts about qemu dirty bitmaps.

Bitmap can be either in active or disable state. In disabled state it does not
get changed on guest writes. And oppositely in active state it tracks guest
writes. This implementation uses approach with only one active bitmap at
a time. This should reduce guest write penalties in the presence of
checkpoints.

And this give us great possibility to store disabled bitmaps in qcow2, not in RAM

  So on first snapshot we create bitmap B_1. Now it tracks changes
from the snapshot 1. On second snapshot we create bitmap B_2 and disable bitmap
B1 and so on. Now bitmap B1 keep changes from snaphost 1 to snapshot 2, B2
- changes from snaphot 2 to snapshot 3 and so on. Last bitmap is active and
gets most disk change after latest snapshot.

Getting changed blocks bitmap from some checkpoint in past till current snapshot
is quite simple in this scheme. For example if the last snapshot is 7 then
to get changes from snapshot 3 to latest snapshot we need to merge bitmaps B3,
B4, B4 and B6. Merge is just logical OR on bitmap bits.

Deleting a checkpoint somewhere in the middle of checkpoint sequence requires
merging correspondent bitmap to the previous bitmap in this scheme.

We use persitent bitmaps in the implementation. This means upon qemu process
termination bitmaps are saved in disks images metadata and restored back on
qemu process start.

note, that it currently works only for qcow2 disks

  This makes checkpoint a persistent property that is we
keep them across domain start/stops. Qemu does not try hard to keep bitmaps.
If upon save something goes wrong bitmap is dropped. The same is applied to the
migration process too. For backup process it is not critical. If we don't
discover a checkpoint we always can make a full backup. Also qemu provides no
special means to track order of bitmaps. These facts are critical for
implementation with one active bitmap at a time. We need right order of bitmaps 
upon
merge - for snapshot N and block changes from snanpshot K, K < N to N we need
to merge bitmaps B_{K}, ..., B_{N-1}. Also if one of the bitmaps to be merged
is missing we can't calculate desired block changes too.

[...]


*Restore operation nuances*

As it was written above to restore a domain one needs to start it in paused
state, export domain's disks and write them from backup. However qemu currently 
does
not let export disks for write even for a domain that never starts guests CPU.
We have an experimental qemu command option -x-vz-nbd-restore (passed together
with -incoming option) to fix it.

Yes. "-incoming x-vz-nbd-restore", it works like -incoming defer, but without logic related to qmp migrate-incoming. So, it is just starting a vm in paused mode with inactive disks. Then we can call qmp cont to resume.


*Links*

[1] Previous version of RFC
    https://www.redhat.com/archives/libvir-list/2017-November/msg00514.html


--
Best regards,
Vladimir

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