[OpenAFS] Understanding questions backup volume

[Hendrik Hoeth]

Hi Lars,

Thus spake Lars Schimmer (l.schimmer@cgv.tugraz.at):
> But: Where is the difference between RO copies and a backup volume?

a backup volume can only exist on the same partition as the RW volume,
since they use 'copy on write' to save the differences between the RW at
the time you updated (vos backup) your backup volume and the current
time. So the backup volume doesn't consume disk space at first, and then
grows as the data in the volume changes.

We use the backup volume for the users to be able to access their files
as of the day before and for our backup (vos dump of the backup volume).

> And if I vos dump the backup volumes to a backup server (amanda-afs or
> just plain dump) I could rebuild the backup volumes. Does this help me
> in case of a lost RW volume?

You can vos restore the dumped backup volume into a RW volume.

> At least a RO copy could be converted to a RW volume in nearly NO
> time, but a backup volume?

The backup volume itself not. You have to restore the dump.

> Our cell is designed to have a RO copy of every RW volume.
> And if one RO copy of a RW volume resist on a file server housed in a
> datacenter "far away" I've got a quick and easy 1-day-backup in case of
> big error here. With the ROtoRW convert the cell is back up very fast.
> So why use backup volumes?

They are tiny ('copy on write') and convertROtoRW has only recently been
added to openafs.

> Are backup volumes built incremental?
> 
> Because with only RO copies, I get a 1-day-backup, but I need a
> long-term-backup with incremental backups.

Dump your ROs. vos dump can do incremental dumps.

Cheers,

   Hendrik

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