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

[Stefan Ring]

> Is the LVM volume aligned to the RAID stripe?  Is their a partition atop
> the RAID LUN and under LVM?  Is the partition aligned?  Why LVM anyway?

Yes, it is aligned. I followed the advice from
<http://www.mysqlperformanceblog.com/2011/06/09/aligning-io-on-a-hard-disk-raid-the-theory/>.

Why LVM? Because we use it on lots of servers, and there is some value
to having a somewhat similar setup in development as in production.
I’ve done similar tests time and again with LVM and without, and I’ve
never ever measured a significant difference. I haven’t re-tested it
this time, true, but I would be surprised if it would magically behave
completely differently this time.

> The devil is always in the details.  Were you using partitions and LVM
> with the RAID1 concat tesing?  With the free space testing?

I used LVM linear for the concatenation – one volume group made from 3
physical volumes. The pvols were on primary partitions. The one-volume
RAID 6 is set up similarly; from only one pvol of course.

> I assumed you were directly formatting the LUN with XFS.  With LVM and
> possibly partitions involved here, that could explain some of the
> mediocre performance across the board, with both EXT4 and XFS.  If one
> wants maximum performance from their filesystem, one should typically
> stay away from partitions and LVM, and any other layers that can slow IO
> down.

I don’t want maximum performance, I want acceptable performance ;).
This means, I am satisfied with 80% or more of what’s possible, but
I’m not satisfied with 15%.