[OpenAFS] Advice on using BTRFS for vicep partitions on Linux

spacefrogg-openafs@spacefrogg.net spacefrogg-openafs@spacefrogg.net
Wed, 22 Mar 2023 15:08:09 +0100 (GMT+01:00)

> At what level does OpenAFS implement CoW?=C2=A0 Is it implemented at
> whole-file-level, i.e. changing a file that is part of a replicated /
> backup volume it will copy the entire file, or is it implemented at
> some range or smaller granularity level (i.e. it will change only that
> range, but share the rest)?

I don't know. I suppose at the file level.

> Can one force OpenAFS to do a verification of these checksums and
> report back any issues?

That is what happens by default. FileLog and SalvageLog should have info on=
 that. In case of failure detection, the volume is taken offline (not taken=
 online again) which manifests in access errors.

> What kind of checksums are these?=C2=A0 Cryptographic ones like
> MD/SHA/newer or CRC-ones?

I don't know. I would try investigating in the direction of the fileserver =
and salvager documentation.

> Granted, RAID is not a backup solution, but it should instead protect
> one from faulty hardware.=C2=A0 Which is exactly what it doesn't do 100%,
> because if one of the drive in the array returns corrupted data, the
> RAID system can't say which one is it (based purely on the returned
> data).=C2=A0 Granted, disks don't just return random data without any oth=
> failure or symptom.

If you have faulty hw, only a backup and new hardware will save you, but do=
 what you must.

> With regard to file-system scrubbing, to my knowledge, only those that
> actually have checksumming can do this, which currently is either

You only lose data checksumming. Metadata checksumming (and CoW for metadat=
a changes) is still used and gives you most of the relevant properties (bec=
ause AFS does its own data checksumming).

> I think that barriers have other implications especially to journaled
> file-systems.

They don't. The ones you have in mind relate to local devices. An NAS will =
simply always report write success as soon as possible (for the sake of cov=
ering up the huge network latencies). Your local FS driver will never know =
the truth... This said, a journaling FS can be less safe over network than =
a non-journaling FS.

> This is true.=C2=A0 It is true even of OpenAFS backup volumes.=C2=A0 :)

That's not true. AFS knows about its backup volumes but not about BTRFS sna=
pshots. At least in principle...

Kind regards,