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Re: [libvirt] RFC backup API

21.03.2016 13:36, Daniel P. Berrange пишет:
On Mon, Mar 21, 2016 at 01:18:19PM +0300, Maxim Nestratov wrote:
Hi all,

It's been already quite a long time since qemu implemented QMP
"drive-backup" command to create block devices backups. Even more, since
qemu 2.4 there is a possibility to create incremental backups. Though it is
possible to backup all attached to a domain disk drives by combining them
into a single QMP transaction command, this way of creating them, not to
mention managing, remains inconvenient for an end user of libvirt. Moreover,
creating a single drive backup via QMP interface isn't handy either. That
said, it looks reasonable to introduce a *new backup API* based on QMP
"drive-backup" facilities.

Though we can start from a single simple function, allowing to create a disk
backup by means of QMP "drive-backup" command, I'd like to discuss here the
level of management libvirt could provide for backup operations. To begin
with, here is the preliminary list of possible functions that I think make
sense for libvirt API.

virDomainCreateBackup - which creates a backup full/incremental of
all/selected disks,
virListBackups - which lists all backups created for a particular
virRestoreBackup - which restores all/selected disks from a backup,
virDeleteBackup - which deletes all/selected disks from a backup.

It looks like backup management functions,  except create one, shouldn't be
or might not be bound to a particular domain and we could possibly leverage
storage pool API with some extension. Specifically, volume definition could
be extended with necessary meta data related to backups.

The *question* is: if the whole idea about this new API as described above
or something similar makes sense?
Well we certainly don't want applications going directly to the QMP
monitor command to achieve this. So given demand for this kind of
feature, some kind of facility will be needed in libvirt. The question
is just what any API would look like.

If yes, then let's find out requirements for it (if any) and I will try to
prepare a patch set with the first RFC implementation to discuss the API in
more details. Looking forward for your opinions on the matter.
Have you researched what VMWare/Hyper-V/VirtualBox APIs support in this
area ? If they have any existing models, it'd be desirable to look at
them to align our APIs where appropriate.

As far as I know VirtualBox doesn't have any backup API, at least there is no reference to it at [1]. What I know about VMWare is that they use an approach that involves temporary snapshot creation, providing an access to quiesced disk data and then deleting this temporary snapshot [2]. Thus, their approach is close to what libvirt can have with 3d party managed backups, i.e. setup an NBD server on qemu side and let 3d party backup software access it from outside.

[1] http://download.virtualbox.org/virtualbox/SDKRef.pdf
[2] http://pubs.vmware.com/vsphere-60/index.jsp#com.vmware.vddk.pg.doc/vddkBkupVadp.9.2.html?path=7_5_0_6_0_0#1014219

Also how does this fit in with recent discussions on QEMU mailing list
about enabling 3rd party vendor backup software by having QEMU expose an
NBD server to do efficient sparse backups. That suggested a very different
kind of API where libvirt would not manage the backups, but just provide
an API to allow an NBD target to be enabled for the 3rd party to manage.

If you mean this thread [3] then as far as I understand, there is an agreement that NDB protocol gets extended a bit in such a way that external clients are able to get dirty block bitmap and this shouldn't be exposed in libvirt API.

For such scenarios libvirt can have the following calls:


When an external backup is started, qemu starts NBD server, allocates CBT (changed block tracking) bitmap and returns allocated NBD port to the caller. After that 3d party backup software is responcible for reading appropriate portion of data according to requested CBT bitmap.

[3] http://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg03075.html


Thus, we have two ways of creating backups. The first is external (3d party managed), which is backed by QMP "blockdev-backup" command, which doesn't need from my point of view more than two calls mentioned above. The second is a another one, which is backed by QMP "drive-backup" command. My first letter covered only the latter case and I believe that this backup management API will be very useful. What is unclear still is: if there should be disk backup facilities only or whole VM backup as well, should libvirt be able to restore deleted VMs, should it be able to list created backups and so on. As for me, using storage pools with some extensions of storage pool API could perfectly solve this task. What do you think?


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