[PVE-User] PBS : is dirty-bitmap really accurate ?

Eneko Lacunza elacunza at binovo.es
Thu Jul 23 08:20:45 CEST 2020


Hi Daniel,

El 22/7/20 a las 18:40, Daniel Berteaud escribió:
> I've started playing with PBS on some VM. So far, it's looking really promizing.
> There's one strange thing though : the percent of the dirty data. For example, I backup one VM every 2 or 3 days. It's a moderately busy server, mainly serving a MariaDB database (zabbix server + mariadb). On each backup, I get similar dirty values :
>
> INFO: using fast incremental mode (dirty-bitmap), 492.7 GiB dirty of 590.0 GiB total
>
> While I'm sur not even 10% of this has really been written.
>
> Get more or less the same problem on other VM. One which I know just sleep all day (my personnal OnlyOffice document server), and which I backup daily, and get values like :
>
> INFO: using fast incremental mode (dirty-bitmap), 5.0 GiB dirty of 10.0 GiB total
>
> Or another small one (personnal samba DC controler) :
>
> INFO: using fast incremental mode (dirty-bitmap), 13.0 GiB dirty of 20.0 GiB total
>
> The only write activity for those 2 are just a few KB or maybe MB of log lines. Respectivly 5 and 13GB of dirty blocks seems unreal.
>
> Am I the only one seeing this ? Could the dirty-bitmap mark dirty blocks without write activity somehow ?
>
I have just checked our test backups. We're using encryption and log 
seems a bit different, but I get the following values for 3 VMs:

INFO: VM Name: dns
INFO: include disk 'scsi0' 'proxmox_r3_ssd:vm-110-disk-0' 5G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/110/2020-07-22T20:00:01Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: enabling encryption
[...]
INFO: backup was done incrementally, reused 8.00 MiB (0%)
INFO: transferred 5.00 GiB in 61 seconds (83.9 MiB/s)

INFO: VM Name: monitor
INFO: include disk 'scsi0' 'proxmox_r3_ssd:vm-124-disk-0' 10G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/124/2020-07-22T20:01:03Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: enabling encryption
[...]
INFO: backup was done incrementally, reused 2.61 GiB (26%)
INFO: transferred 10.00 GiB in 63 seconds (162.5 MiB/s)

INFO: VM Name: monitor2
INFO: include disk 'scsi0' 'proxmox_r3_ssd:vm-149-disk-0' 10G
INFO: backup mode: snapshot
INFO: ionice priority: 7
INFO: creating Proxmox Backup Server archive 'vm/149/2020-07-22T20:02:06Z'
INFO: issuing guest-agent 'fs-freeze' command
INFO: enabling encryption
[...]
INFO: backup was done incrementally, reused 112.00 MiB (1%)
INFO: transferred 10.00 GiB in 69 seconds (148.4 MiB/s)

All those servers have very low disk activity (monitors are nagios servers).

 From the logs I'd say incremental backup only reused (saved) 0%, 26% 
and 1% of backup size... but maybe log is wrong... ¿?

Cheers
Eneko


-- 
Eneko Lacunza                   | Tel.  943 569 206
                                 | Email elacunza at binovo.es
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