Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
Hi all, We use DRBD on top of LVM - so for every replicated block device we need to create, we create an LVM logical volume on each server and then create a DRBD resource on top of that LV. Each LV/DRBD is a volume for a VM. Whilst dual-primary capability is enabled, this is only enabled briefly during VM live migrations; the rest of the time, each resource runs in primary/secondary mode. This has all been working well. We now want to extend this to take snapshot backups with restoration. If we snapshot the LV itself, presumably we'll get a copy not only of the VM's volume block data but also of DRBD metadata on the LV (which I understand is stored in the end/final segment(s) of the LV). Questions: (1) Is there any danger of this process not giving a DRBD-consistent snapshot? Is there anything within DRBD which needs to be flushed/synced prior to starting the snapshot (as is the practice with SQL)? (2) How is best to restore an image file of the LV, assuming the VM is offline and, if necessary, the DRBD resource can be detached/disconnected on each/both side(s)? (3) Presumably it doesn't matter if the snapshot is taken on the primary or secondary? The concern is that doing a dd to overwrite the underlying volume, which includes rolling back the DRBD metadata, may confuse DRBD somehow. Particularly if it's not done in the right way - e.g. if the secondary is left connected and only the primary LV is overwritten, the secondary will hold newer data and the sync may occur in the wrong direction (as, unusually, we want an older version to replace the newer one). Can anyone with a knowledge of the workings of metadata and the sync process advise what would be a correct procedure for underlying-LV snapshot restoration? I've tried to research this but not been able to find anything easily digestible. Second related topic: if we've got an existing snapshot, can we use this as the basis for creating a totally new DRBD resource... i.e. creating a new LV of identical size, dd the image to the LV and then create a new DRBD resource on top. Is there anything in the metadata (like global UUIDs) which could be duplicated and confuse DRBD? If so, is there a recommended way to "reset" the metadata prior to attaching so it will appear as a totally new pre-seeded volume? Thanks for any input, Charles.