Re: {Possible Spam} RE: stripe_cache_active always 0

[Benjamin ESTRABAUD <be@mpstor.com>]

On 07/01/16 16:34, Robert Kierski wrote:
> Adam,
>
> stripe_cache_active is the value to indicate how many pages are currently in use in the stripe cache.  I could be wrong about this... but I don’t think you can disable the stripe cache.  You can shrink it to an unreasonable size, but the minimum you can set stripe_cache_size to is 17 (pages).
>
> Having said that, it may be that your RAID isn't using the stripe cache or that you're not checking the value frequently enough to see stripe cache activity.
>
> A file system that is properly tuned, and doing buffered IO will likely reduce the stripe cache usage.  The stripe cache is basically where data gets put when the RAID is doing partial stripe IO's, calculating parity, or waiting for slow devices to complete their IO's.  It's not intended to be yet another level of cache, so it gets purged when the RAID is done with it.  So, unlike other caches that will hold onto data until there is memory pressure, the stripe cache is more likely to be empty.
>
> As far as adjusting stripe_cache_size... The stripe cache is dynamically allocated.  It won't save any RAM by decreasing stripe_cache_size.  Decreasing it too much will negatively impact performance as it's more likely you'll do Read-Modify-Write's when doing partial stripe writes.
>
I find that tuning it a bit higher (but not too high) has a very 
positive impact on rebuild times (where we are therefore building 
stripes). I can't remember exactly the values but we were setting it to 
256 or 512 or even 1024 while building RAID5 or 6 with positive results.

Regards,
Ben.

> Bob Kierski
> Senior Storage Performance Engineer
> Cray Inc.
> 380 Jackson Street
> Suite 210
> St. Paul, MN 55101
> Tele: 651-967-9590
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