On Wed, Mar 22, 2023 at 10:30 AM <spacefrogg-open...@spacefrogg.net> wrote: > OpenAFS implements its own CoW and using CoW below that again has no benefits > and disturbs the fileservers "free-space" assumptions. It knows when it makes > in-place updates and does not expect to run out of space in that situation.
At what level does OpenAFS implement CoW? 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'm asking this because I've assumed (based on empirical observations) that all files stored in OpenAFS (via the proper `afsd`) will end somewhere in `/vicepX` as individual files. (I.e. if I were to `md5sum` all the files from `/afs/some-cell`, and then `md5sum` all the files in `/vicepX`, then the first set of `/afs/...` is a subset of the second one `/vicepX`.) > > Unfortunately (at least for my use-case) losing the checksumming and > > compression is a no-go, because these were exactly the features that > > made BTRFS appealing versus Ext4. > > If you say so... > AFS does its own data checksumming. Can one force OpenAFS to do a verification of these checksums and report back any issues? What kind of checksums are these? Cryptographic ones like MD/SHA/newer or CRC-ones? > > Also, regarding RAID scrubbing, it doesn't cover the issue of > > checksumming, because (for example with RAID5) it can only detect that > > one of the disks has corrupted data, but couldn't say which. > > Do not use RAID to prevent data loss! That's what backups are for. RAID is > for operative redundancy. Scrubbing also tells you about your state of FS > metadata. So, it's not that it has no use without checksumming. I only use > RAID 1 and 1-0. They have lower dataloss probabilities that RAID 5. Granted, RAID is not a backup solution, but it should instead protect one from faulty hardware. 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). Granted, disks don't just return random data without any other failure or symptom. With regard to file-system scrubbing, to my knowledge, only those that actually have checksumming can do this, which currently is either BTRFS or ZFS. > All -sync properties are ineffective with NAS, because the network layer and > far-end OS decide on actual commit strategies. So you might as well stop > deceiving yourself and disable write barriers. I think that barriers have other implications especially to journaled file-systems. > You will use subvolumes the moment you start making snapshots. So be careful > to not deceive yourself. A forgotten snapshot can easily get you into trouble > the moment you move off some volumes to make room for a large addition, just > to realise no space opened up at all. This is true. It is true even of OpenAFS backup volumes. :) Ciprian. _______________________________________________ OpenAFS-info mailing list OpenAFS-info@openafs.org https://lists.openafs.org/mailman/listinfo/openafs-info
