XFS: Abysmal write performance because of excessive seeking (allocation groups to blame?)

[Stefan Ring]

> Let's rerun it with files cached (the machine has 16 GB RAM, so
> every single file must be cached):
>
> # time tar xf test.tar
>
> real    0m50.842s
> user    0m0.809s
> sys     0m13.767s

That’s about the same time I’m getting on a fresh (non-fragmented)
file system with the RAID 6 volume.

Interestingly, the P400’s successor, the P410 does recognize a setting
that the P400 lacks, which is called elevatorsort. It sounds like this
could make all the difference. Unfortunately, the P400 doesn’t have
it. I don’t have a P410 with more than 2 drives to test this, but some
effect should definitely be measurable.

Since this finding has piqued my interest again, I’m willing to invest
a little more time, but I’m completely occupied for the next few days,
so it will have to wait a while.