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

[Jeffrey E Altman]

On 3/22/2023 9:34 AM, Ciprian Craciun (ciprian.crac...@gmail.com) wrote:

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)?

OpenAFS performs CoW on whole files within a Volume on the first update after a Volume clone is created.  The clone can be a ROVOL, BACKVOL or untyped clone.

OpenAFS CoW is limited to sharing a vnode between multiple Volume instances.
Once a vnode can no longer be shared between two or more Volumes, the vnode
is copied.

OpenAFS does not perform CoW at a byte range level.


OpenAFS will copy the entire 1GB.

On Linux btrfs and xfs both support CoW at the block level.   A one byte change to a 1GB file on btrfs will result in one block being copied and modified.   Whereas

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.

OpenAFS does not maintain checksums.  Checksums are neither transmitted in
the RXAFS_FetchData and RXAFS_StoreData RPCs messages nor are checksums
stored and compared when reading and writing to the vice partition.

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.

Bit flips occur more frequently than we would like which was the rationale behind

adding checksums and multiple copies and self-healing to ZFS.

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.

There are some other examples but these are the only two regularly available as

a local Linux filesystem.

Jeffrey Altman

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