Re: RAID stripe size

[Greg Troxel <gdt%lexort.com@localhost>]

Edgar Fuß <ef%math.uni-bonn.de@localhost> writes:

> On a populated FFS, is there an easy way to determine how many
> fragments are in use or how many blocks are split into fragments?

dumpfs might help.    My impression is that files are always the highest
number of blocks that fit, and then fragments as needed.    If the files
are large, there shouldn't be that many fragments.

fsck (-ny while mounted) reports, on an arbitrarily chosen filesystem:

  603427 files, 48107560 used, 11047434 free (184530 frags, 1357863 blocks, 0.3% fragmentation)

> Just in case: What this really is about is chosing the RAID stripe
> unit size.  On a three-component RAID 5, I think I basically have two
> options: make a stripe the size of a fragment or make it the size of a
> block.  The data on the RAID set is a Borg backup, and I can examine
> data on another backup server. Since backups are mostly written to, I
> think I need to optimize for write speed.

Usually block/fragment is 8k/1k, or 16k/2k.   I have the impression
RAID5 stripe sizes tend be more like 32k or 64k.

I am unclear on if ffs tends to allocate consecutive blocks.

You might consider ZFS with raidz1, but you'll have to keep
kern.maxvnodes low to avoid a deadlock with zfs freeing and dirty mmap'd
pages.  With that workaround, my system has been reliable.  Without, it
crashed after a week or so.