On 15/05/17 02:11, Michael Hierweck wrote:
Hi all,
up to now to use LVM (thick) provisioning to provide virtual disks for
VMs: RAID => LVM => LV => DRBD => KVM/QEMU (virtio).
We would like to be able to take snapshots from our virtual disks in
order to be able to use them as readonly source to consistent backups.
Therefore we would need to introduce a LVM thin provisioning layer
somewhere.
A. TP below DRBD
RAID => LVM => LVM-TP => LV => DRBD => KVM/QEMU (virtio)
- simple setup
- I suppose going back to an older snapshost would this break DRBD and
require a complete resync/verify run
This is what I do. When I need to restore a snapshot, I have done the
following:
dd from the snapshot to a temp file elsewhere (or increase the snapshot
size to be larger than the original disk image)
dd from the temp file to the DRBD device, we expect to get a write
error/run out of space at the end, because we can't write the DRBD
portion at the end of the drive.
I haven't attempted it, and it would be interesting to hear the required
process to simply use LVM to rollback to the snapshot, but suspect the
following steps are needed:
1) Stop the DRBD device (both primary and secondary)
2) rollback the LV using LVM tools
3) Start the DRBD device on the primary only
4) Re-initialise the DRBD device on the secondary
5) Start the DRBD device on the secondary
Comments?
Personally, that method doesn't work for me, since I can't "stop" the
DRBD device because it is constantly open (exported via iscsi)
B. TP on top of DRBD
RAID => LVM => LV => DRBD => LVM => LVM-TP => LV => KVM/QEMU (virtio)
- complex setup
- more layers affect reliability and performance
- snapshots are replicated and consistent
- additional diskspace for snaphosts is limited by the DRBD volume size
an connot be shared across volumes.
I would like to discuss the pros and the cons of both approaches while
considering internal vs. external metadata.
--
Adam Goryachev Website Managers www.websitemanagers.com.au
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