On 18.2.2014 02:23, KONDO Mitsumasa wrote:
> Hi,
>
> I don't have PERC H710 raid controller, but I think he would like to
> know raid striping/chunk size or read/write cache ratio in
> writeback-cache setting is the best. I'd like to know it, too:)
We do have dozens of H710 controllers, but not with SSDs. I've been
unable to find reliable answers how it handles TRIM, and how that works
with wearout reporting (using SMART).
The stripe size is actually a very good question. On spinning drives it
usually does not matter too much - unless you have a very specialized
workload, the 'medium size' is the right choice (AFAIK we're using 64kB
on H710, which is the default).
With SSDs this might actually matter much more, as the SSDs work with
"erase blocks" (mostly 512kB), and I suspect using small stripe might
result in repeated writes to the same block - overwriting one block
repeatedly and thus increased wearout. But maybe the controller will
handle that just fine, e.g. by coalescing the writes and sending them to
the drive as a single write. Or maybe the drive can do that in local
write cache (all SSDs have that).
The other thing is filesystem alignment - a few years ago this was a
major issue causing poor write performance. Nowadays this works fine,
most tools are able to create partitions properly aligned to the 512kB
automatically. But if the controller discards this information, it might
be worth messing with the stripe size a bit to get it right.
But those are mostly speculations / curious questions I've been asking
myself recently, as we've been considering SSDs with H710/H710p too.
As for the controller cache - my opinion is that using this for caching
writes is just plain wrong. If you need to cache reads, buy more RAM -
it's much cheaper, so you can buy more of it. Cache on controller (with
a BBU) is designed especially for caching writes safely. (And maybe it
could help with some of the erase-block issues too?)
regards
Tomas