A question about pve-zsync

[giovvv]

Ok, maybe this is a stupid question, but I was wondering...

pve-zsync syncs a snapshot of the zvol containing the VM virtual disk. But since this is done while the VM is running (unlike what vzdump does), isn't it possible that the guest filesystem is "frozen" while it is in some inconsistent state? I am thinking, for example, of what can happen to a database if the VM is restored from such a backup.

 

[guletz]

Hi,

No. When a snapahot is created, zfs will also flush all data from ARC to the disks before start the snapshot.


Good luck / Bafta

 

[Toxik]

Will pve-zsync freeze the guest file system via the guest agent? https://wiki.qemu.org/Features/Snapshots#Guest_Agent

 

[guletz]

Will pve-zsync freeze the guest file system via the guest agent?

No. But for zfs you do not need guest-freeze.

 

[Toxik]

No. But for zfs you do not need guest-freeze.

But the Qemu wiki syays something else:

Note that guest agent collaboration is also needed for snapshots using other methods, such as snapshots performed on btrfs, LVM, enterprise storage, etc.

 

[fabian]

No. But for zfs you do not need guest-freeze.

that's not true. ZFS snapshots are internally consistent, since ZFS always is, even on powerloss. but that doesn't mean that a freeze via the agent does not trigger additional actions that help with consistency within the VM (e.g., syncing the disks from a guest PoV, hooking into OS-provided consistency mechanisms, you could even call arbitrary commands to settle in-flight DB operations or whatever else strikes your fancy )

 

[guletz]

I start using snapshots in zfs 8 years ago. And usualy I need to recover files from this snapshots daily(including here DB files). I NEVER,
ever see any problem after restore this files!

 

[fabian]

Hi,

No. When a snapahot is created, zfs will also flush all data from ARC to the disks before start the snapshot.


Good luck / Bafta

that is also only partially true ("flushing the ARC" does not make much sense). any transactions in-flight when issuing a snapshot will be persisted before the snapshot is actually done, since a snapshot also happens in a (later) transaction .

 

[fabian]

I start using snapshots in zfs 8 years ago. And usualy I need to recover files from this snapshots daily(including here DB files). I NEVER,
ever see any problem after restore this files!

if all your applications are well-designed, you shouldn't. not all applications are though

 

[guletz]

if all your applications are well-designed, you shouldn't. not all applications are though

This means that I have had a lots of lucks during last 8 years

 

[guletz]

ANY

in-flight when issuing a snapshot will be persisted

IF the data from ARC is flushed to the disk. I do not see any other case

 

[Toxik]

I start using snapshots in zfs 8 years ago. And usualy I need to recover files from this snapshots daily(including here DB files). I NEVER,
ever see any problem after restore this files!

Well, I guess most DBs should be power-off save, as long as the underlying FS does not do any mess.

 

[Toxik]

that's not true. ZFS snapshots are internally consistent, since ZFS always is, even on powerloss. but that doesn't mean that a freeze via the agent does not trigger additional actions that help with consistency within the VM (e.g., syncing the disks from a guest PoV, hooking into OS-provided consistency mechanisms, you could even call arbitrary commands to settle in-flight DB operations or whatever else strikes your fancy )

Still the question: does pve-zsync freeze the guest FS before snapshot?