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

[Jeffrey E Altman]

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On 3/22/2023 3:47 PM, spacefrogg-openafs@spacefrogg.net wrote:
>> 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.
> Thanks for clearing this up. So, volume inconsistencies are just detected on the metadata level?

The salvager can update Volume metadata to be consistent with what is 
stored in the vice partition, it can attach orphaned vnodes, it can 
remove directory entries for missing vnodes, it can correct vnode size 
information,  it can rebuild directories, and a few other things.   
However, there is no ability to repair a damaged block or restore a 
missing vnode.

btrfs and zfs have benefits in being able to recover from damaged disk 
sectors by maintaining multiple copies of each data block (if that 
functionality is configured.)   However, they each have downsides as 
well.   Both require vasts sums of memory compared to other file 
systems.   Both have very inconsistent performance characteristics 
especially as free space falls below 40% and as memory pressure 
increases.   Both can fail under memory pressure or when their storage 
volumes are full.

AuriStor recommends that AuriStorFS vice partitions be deployed using 
xfs.   There are severalFS AuriStor end users that build xfs filesystems 
on zfs exported block devices when zfs is desired for additional 
reliability.

If zfs is going to be installed on Linux I recommend using a Linux 
distribution that packages zfs to ensure that an incompatible kernel 
update is never issued.  I have observed sites lose vice partitions 
hosted using zfs of rhel because of subtle kernel incompatibilities.  A 
risk with all out-of-tree filesystems that are not tested against every 
released kernel version but especially with out-of-tree non-GPL filesystems.

Jeffrey Altman




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