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

[Stefan Ring]

> http://hardforum.com/showpost.php?s=c19964285e760bee47b8558ae82899d5&p=1033958051&postcount=4
>  «One is a stick of memory with a battery attached to it and one
>  without. The one without is what the basic models ship with
>  and usually has either 256 or 512Mb of memory, it supports
>  caching for read operations only. [ ... ] You need the battery
>  backed write cache module if you want to be able to use/turn
>  on write caching on the array controller which makes a huge
>  difference for write performance in general and is pretty much
>  critical for raid 5 performance on writes.»
>
> It may be worthwhile to check if there is an enabled BBWC
> because if there is the BBWC the host adapter should be
> buffering writes up to 256MiB/512MiB and sorting them thus long
> inter-AG seeks should be happening only 10 or 5 times or not
> much more (4 times) that. Instead it may be happening that the
> P400 is doing write-through, which would reflect the unsorted
> seek pattern at the Linux->host adapter level into the host
> adapter->disk drive level.

The write cache can also be enabled without a battery present (at
considerable risk), but I insisted to get a battery. It is enabled,
and it makes a noticeable difference. Without it, it’s even slower
(more than factor 2).