[libvirt] [RFC v3] external (pull) backup API

[Eric Blake]

On 05/21/2018 10:52 AM, Vladimir Sementsov-Ogievskiy wrote:
> 18.05.2018 01:43, Eric Blake wrote:
>> Here's my updated counterproposal for a backup API.
>>
> 
> [...]
> 
>>
>> Representing things on a timeline, when a guest is first created,
>> there is no dirty bitmap; later, the checkpoint "check1" is created,
>> which in turn creates "bitmap1" in the qcow2 image for all changes
>> past that point; when a second checkmark "check2" is created, a qemu
>> transaction is used to create and enable the new "bitmap2" bitmap at
>> the same time as disabling "bitmap1" bitmap.  (Actually, it's probably
>> easier to name the bitmap in the qcow2 file with the same name as the
>> Checkpoint object being tracked in libvirt, but for discussion
>> purposes, it's less confusing if I use separate names for now.)
>>
>> creation ....... check1 ....... check2 ....... active
>>         no bitmap       bitmap1        bitmap2
>>
>> When a user wants to create a backup, they select which point in time
>> the backup starts from; the default value NULL represents a full
>> backup (all content since disk creation to the point in time of the
>> backup call, no bitmap is needed, use sync=full for push model or
>> sync=none for the pull model); any other value represents the name of
>> a checkpoint to use as an incremental backup (all content from the
>> checkpoint to the point in time of the backup call; libvirt forms a
>> temporary bitmap as needed, the uses sync=incremental for push model
>> or sync=none plus exporting the bitmap for the pull model).  For
>> example, requesting an incremental backup from "check2" can just reuse
>> "bitmap2", but requesting an incremental backup from "check1" requires
>> the computation of the bitmap containing the union of "bitmap1" and
>> "bitmap2".
> 
> I have a bit of criticism on this part, exactly on ability to create a 
> backup not from last checkpoint but from any from the past. For this 
> ability we are implementing the whole api with checkpoints, we are going 
> to store several bitmaps in Qemu (and possibly going to implement 
> checkpoints in Qemu in future). But personally, I don't know any real 
> and adequate use cases for this ability.
> 
> I heard about the following cases:
> 1. Incremental restore: we want to rollback to some point in time (some 
> element in incremental backup chain), and don't want to copy all the 
> data, but only changed.
> - It's not real case, because information about dirtiness is already in 
> backup chain: we just need to find allocated areas and copy them + we 
> should copy areas, corresponding to dirty bits in active dirty bitmap in 
> Qemu.

If you do a pull mode backup (where the dirty bitmaps were exported over 
NBD), then yes, you can assume that the third-party app reading the 
backup data also saved the dirty bitmap in whatever form it likes, so 
that it only ever has to pull data from the most recent checkpoint and 
can reconstruct the union of changes from an earlier checkpoint offline 
without qemu help.  But for a push mode backup (where qemu does the 
pushing), there is no way to expose the dirty bitmap of what the backup 
contains, unless you backup to something like a qcow2 image and track 
which clusters in the backup image were allocated as a result of the 
backup operation.  So having a way in the libvirt API to grab an 
incremental backup from earlier than the most recent checkpoint may not 
be needed by everyone, but I don't see a problem in implementing it either.

> 
> 2. Several backup solutions backing up the same vm
> - Ok, if we implement checkpoints, instead of maintaining several active 
> dirty bitmaps, we can have only one active bitmap and others disabled, 
> which lead to performance gain and possibility to save RAM space (if we 
> unload disabled bitmaps from RAM to qcow2). But what are real cases? 
> What is the real benefit? I doubt that somebody will use more than 2 - 3 
> different backup providers on same vm, so is it worth implementing such 
> a big feature for this? It of course worth doing if we have 100 
> independent backup providers.
> Note: the word "independent" is important here. For example it may be 
> two external backup tools, managed by different subsystems or different 
> people or something like this. If we are just doing a backup weekly + 
> daily, actually, we can synchronize them, so that weekly backup will be 
> a merge of last 7 daily backups, so weekly backup don't need personal 
> active dirty bitmap and even backup operation.

I'm not sure if this is a complaint that libvirt should allow more than 
one active bitmap at a time, vs. having exactly one active bitmap at a 
time and then reconstructing bitmaps over larger sequences of time as 
needed.  But does that change the API that libvirt should expose to end 
users, or can it just be an implementation detail?

> 
> 3. Some of backups in incremental backup chain are lost, and we want to 
> recreate part of the chain as a new backup, instead of just dropping all 
> chain and create full backup.
> In this case, I can say the following:
> disabled bitmaps (~ all checkpoints except the last one) are constant 
> metadata, related to the backup chain, not to the vm. And it should be 
> stored as constant data: may be on the same server as backup chain, 
> maybe on the other, maybe in some database, but not in vm. VM is a 
> dynamic structure, and I don't see any reason of storing (almost) 
> unrelated constant metadata in it. Also, saving this constant 
> backup-related metadata separately from vm will allow to check it's 
> consistency with a help of checksums or something like this. Finally, 
> I'm not a specialist in storing constant data, but I think that the vm 
> is not the best place.

How does backup bitmap data get lost?  If the image is only managed by 
libvirt, then libvirt shouldn't be losing arbitrary bitmaps.  If a 
third-part entity is modifying the qcow2 images (presumably while the 
guest is offline, as editing a file that is simultaneously in use by 
qemu is a no-no), then all bets are off anyways, as you really shouldn't 
be trying to independently manage qcow2 files that are already being 
managed by libvirt.

> 
> Note: Hmm, do someone have real examples of such user cases? Why backups 
> are lost, is it often case? (I heard an assumption, that it may be a 
> tool, checking backups (for example create a vm over the backup and 
> check that it at least can start), which is running in background. But 
> I'm not sure, that we must drop backup if it failed, may be it's enough 
> to merge it up)

I'm more worried about the implementation of checkpoints and dirty 
bitmaps across domain snapshots (where we have to consider copying one 
or more bitmaps from the base image to the active image when creating a 
snapshot, and conversely about merging bitmaps from the active image 
into the base when doing a live commit).

> 
> 3.1 About external backup: we have even already exported this metadata 
> to the third backup tool. So, this tool should store this information 
> for future use, instead of exporting from Qemu again.
> 
> To summarize:
> 1. I doubt that discussed ability is really needed.
> 2. If it is needed, I doubt that it's a true way to store related 
> disabled bitmaps (or checkpoints) in Qemu.

So, to make sure I understand, the only thing that you are debating 
whether we need is the ability to grab a backup image from earlier than 
the most-recent checkpoint?  Remember, the proposal is whether we have a 
sufficiently powerful libvirt API to cover multiple use cases, even if 
not all users need all of the permutations of use cases, while still 
being something that is concise enough to document and implement on top 
of existing qemu semantics.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org