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

[Stefan Ring]

On Sat, Apr 14, 2012 at 9:32 AM, Stan Hoeppner <stan at hardwarefreak.com> wrote:
> On 4/13/2012 2:36 PM, Stefan Ring wrote:
>>> 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.
>>
> What configuration are you running right now Stefan?  You said you went
> back to XFS due to the EXT4 lockups, but I can't recall what RAID config
> you put underneath it this time.

RAID 6 4+2, LVM (single volume), 32kb stripe size (=> full stripe:
128kb), agcount=4

Except for the stripe size, the same config I had originally. The only
instance of really poor behavior is with the (artificially) fragmented
free space.

I have moved everything elsewhere for a while, so I can once again do
some testing that involves destroying and rebuilding everything.