[SOLVED] Ceph - Can't read/write after a live disk snapshot

Community Support
1

I successfully made a live disk snapshot on a running Linux VM with qemu-guest-agent installed :

Sat Dec 19 15:51:37 2015 [Z0][VM][I]: New LCM state is RUNNING
Sat Dec 19 15:51:56 2015 [Z0][VM][I]: New state is ACTIVE
Sat Dec 19 15:51:56 2015 [Z0][VM][I]: New LCM state is DISK_SNAPSHOT
Sat Dec 19 15:51:57 2015 [Z0][VMM][I]: Successfully execute transfer manager driver operation: tm_snap_create_live.
Sat Dec 19 15:51:57 2015 [Z0][VMM][I]: VM disk snapshot successfully created.
Sat Dec 19 15:51:57 2015 [Z0][VM][I]: New LCM state is RUNNING
Sat Dec 19 15:51:57 2015 [Z0][LCM][I]: VM disk snapshot operation completed.

But after the snapshot, if I try to reboot the system using the reboot command in the guest system, my SSH session closes but the system won’t reboot and never comes back online. If I don’t try to reboot the system and just try to access the disk (read or write, ls, cat, touch, …), my session freezes.

I don’t have this problem if I STOP or SUSPEND the VM before doing the disk snapshot.

Is the live disk snapshot with Ceph backend is possible?

Thank you.
ONE 4.14.2 with Ceph 0.94.5

2

Yes it is supported, note that if you do not revert the operation is just a
rbd snapshot operation on the disk, that should not alter the status of the
disk image … A couple of things to check:

1.- If you updated from a previous version, perform a onehost sync
2.- Maybe you can try to install qemu-guest-agent, the driver is actually
trying a do a domfsfreeze.

Cheers

3

Thanks for your quick help.

The qemu-guest-agent is already installed and running. The hosts are also synced.

Note that I’m trying to snapshot the system disk (vda, ID-0). Maybe the Ceph live disk snapshot is only possible on non-system target disk?

4

I solved this problem by setting Cache=writethrough for the storage disk 0 (system disk). I can now live snapshot the disk without freezing the system. Even with the qemu-guest-agent running, the system freezed after a disk snapshot if the disk Cache option is left blank.

…as explained in the doc :

Warning
Depending on the CACHE the live snapshot may or may not work correctly. For more security use CACHE=writethrough although this delivers the slowest performance.

5

Thanks for letting us know!