About snapshots and locked VM's

[stuba]

Hi,

have I got idea of snaphost completely wrong here? I've been doing backups for one Windows server that holds about 600gb of data with lzo compression. Windows server has qcow2 disks.

How I have allways understood snapshot is that snapshot holds all data and status for certain moment in time. This snapshot is used to take backup, snapshot of course will contain RAM status. Now it seems that current snapshot locks whole server for whole time snapshot is beeing processed and releases lock afterwards?

I could not find any good documentation for this. I just would like to know is there somenthig wrong with our system or is this just how it works.

 

[dietmar]

Now it seems that current snapshot locks whole server for whole time snapshot is beeing processed and releases lock afterwards?

No. But what kind of snapshot do you talk about?

 

[stuba]

Just regular backups with mode snapshot.

 

[dietmar]

VM vzdump snapshots works as you described - they do not block anything.

 

[gkovacs]

How I have allways understood snapshot is that snapshot holds all data and status for certain moment in time. This snapshot is used to take backup, snapshot of course will contain RAM status. Now it seems that current snapshot locks whole server for whole time snapshot is beeing processed and releases lock afterwards?

Snapshot is simply a state of the disk at a specified time (has nothing to do with RAM).

Proxmox uses snapshot feature of the storage subsystem to take backups in a consistent state (meaning that no disk writes are half-complete).

 

[stuba]

Ok thanks for this info.

Now server is working just fine while running backup. About same time backup started server was unresponsive for a while. Might have just been to hasty or something else was causing problems.

 

[spirit]

Proxmox uses snapshot feature of the storage subsystem to take backups in a consistent state (meaning that no disk writes are half-complete)..

No, that's wrong.

for QEMU
----------
qemu-kvm backup don't use any snapshot feature of storage system. (It's works with any storage).
Maybe the "backup snapshot" name is confusing.
When the backup start, a disk blocks map is created in memory (you can see this as a snapshot/capture of the disk), and when backup occur, if new writes are coming to the disk at the same time, writes are going to the backup and the disk vm.
(and this dirty block map is updated to known which blocks are already backuped).

For OPENVZ
------------
in this case, lvm snapshot are used. It's works only with lvm as storage.

 

[m.ardito]

qemu-kvm backup don't use any snapshot feature of storage system. (It's works with any storage). Maybe the "backup snapshot" name is confusing. When the backup start, a disk blocks map is created in memory (you can see this as a snapshot/capture of the disk), and when backup occur, if new writes are coming to the disk at the same time, writes are going to the backup and the disk vm. (and this dirty block map is updated to known which blocks are already backuped). For OPENVZ ------------ in this case, lvm snapshot are used. It's works only with lvm as storage.

interesting, I never realized this...

http://pve.proxmox.com/wiki/Backup_and_Restore

• snapshot (KVM): Use KVM live backup (no downtime, online)
• snapshot (OpenVZ): Use LVM2 snapshots (no downtime, online)

And about "KVM live backup" => http://wiki.qemu.org/Features/Livebackup

• "does not transfer the memory contents of the running VM, or the device states of emulated devices, i.e. livebackup is not VM suspend"

Marco

 

[ktecho]

Excuse me if I get to life an old thread, but I cannot find useful info about this. I have 4 kvm VMs and whenever I make or delete an snapshot, VMs get completely locked. They don't accept connections, ssh drops if I touch any key, etc. I mean, it's not "live" by any means.

I'm using qcow2 and latest proxmox packages. Any clue on what's happening?