About the dirty-bitmaps

[efernandez]

Hi all.

A question about the dirty-bitmaps of the disks. Can the dirty-bitmaps be lost if two consecutive backups are made but to different datastores? I'll explain:

We have a virtual machine with a scheduled backup every hour on a storage called PBS-snapshot and another task at 9:00 p.m. on a different storage called PBS. The virtual machine did not power off / power on.

Tasks logs:

DATASTORE: PBS-snapshot
INFO: Backup started at 2021-07-15 20:30:00
INFO: status = running
INFO: VM Name: XXX
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive ‘XXX’
INFO: scsi0: dirty-bitmap status: OK (296.0 MiB of 32.0 GiB dirty)
INFO: scsi1: dirty-bitmap status: OK (432.0 MiB of 1.0 TiB dirty)

DATASTORE: PBS
INFO: Backup started at 2021-07-15 21:00:00
INFO: status = running
INFO: VM Name: XXX
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive ‘XXX’
INFO: scsi0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: scsi1: dirty-bitmap status: existing bitmap was invalid and has been cleared

DATASTORE: PBS-snapshot
NFO: Backup started at 2021-07-16 00:16:00
INFO: status = running
INFO: VM Name: XXX
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive ‘XXX’
INFO: scsi0: dirty-bitmap status: existing bitmap was invalid and has been cleared
INFO: scsi1: dirty-bitmap status: existing bitmap was invalid and has been cleared

DATASTORE: PBS-snapshot
INFO: Backup started at 2021-07-16 08:10:53
INFO: status = running
INFO: VM Name: XXX
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive ‘XXX’
INFO: scsi0: dirty-bitmap status: OK (380.0 MiB of 32.0 GiB dirty)
INFO: scsi1: dirty-bitmap status: OK (292.0 MiB of 1.0 TiB dirty)

It seems to me that by changing the datastores we lose the dirty bitmaps, right?

Thanks in advance.

 

[fabian]

yes, the bitmap is only valid for a single backup snapshot in a single datastore, and we only keep one bitmap. so switching the datastore, or removing the corresponding backup snapshot will always invalidate the bitmap.

 

[efernandez]

thanks!!

 

[kocio]

Hi, I'd like to ask what about dirty-bitmap of a stopped machine on PBS 1.1? I guess it should not change, but it looks like it's being recreated each time we do a backup, effectively just wasting the time:

INFO: status = stopped
INFO: backup mode: stop
INFO: ionice priority: 7
INFO: VM Name: *****
INFO: include disk 'scsi0' 'local-lvm:vm-2252-disk-0' 100G
INFO: creating Proxmox Backup Server archive 'vm/2252/2021-07-26T00:00:14Z'
INFO: starting kvm to execute backup task
INFO: enabling encryption
INFO: started backup task '2f98bae9-54e5-4a81-b2b6-ea002ba83f43'
INFO: scsi0: dirty-bitmap status: created new
INFO:   0% (412.0 MiB of 100.0 GiB) in  3s, read: 137.3 MiB/s, write: 0 B/s
[...]
INFO: 100% (100.0 GiB of 100.0 GiB) in 14m 30s, read: 245.0 MiB/s, write: 0 B/s
INFO: backup is sparse: 22.29 GiB (22%) total zero data
INFO: backup was done incrementally, reused 100.00 GiB (100%)
INFO: transferred 100.00 GiB in 870 seconds (117.7 MiB/s)
INFO: stopping kvm after backup task
INFO: Finished Backup of VM 2252 (00:14:34)
INFO: Backup finished at 2021-07-26 02:14:48

 

[fabian]

bitmaps are only kept while a VM is running. once you stop a VM, we cannot be sure that the disk remains unmodified until the next start - if anything modified the disk in the meantime, the backup based on the old bitmap would be corrupt/invalid.

 

[kocio]

I see, I just thought PVE can be sure nothing has changed on disk while the machine is off. Thanks for the explanation, fabian!