Re: stripe_cache_active always 0

[Roman Mamedov <rm@romanrm.net>]

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On Thu, 7 Jan 2016 16:34:36 +0000
Robert Kierski <rkierski@cray.com> wrote:

> As far as adjusting stripe_cache_size... The stripe cache is dynamically allocated.  It won't save any RAM by decreasing stripe_cache_size.

Since when?

# echo 512 > /sys/devices/virtual/block/md0/md/stripe_cache_size
# free
             total       used       free     shared    buffers     cached
Mem:      16159912   15672696     487216      12588         52   14484708
-/+ buffers/cache:    1187936   14971976
Swap:            0          0          0

# echo 32768 > /sys/devices/virtual/block/md0/md/stripe_cache_size
# free
             total       used       free     shared    buffers     cached
Mem:      16159912   15957880     202032      12588         52   14214952
-/+ buffers/cache:    1742876   14417036
Swap:            0          0          0

You can see that's not the case (on kernel 4.3.3 with four-member RAID5);

And it's quite easy to rapidly hit OOM issues on high-member-count arrays by
setting stripe_cache_size to larger values (not realizing that this is *in
pages* not kilobytes or sectors, and *per disk*).

As for the original question, try checking stripe_cache_active e.g. once
per second during heavy write load to the filesystem.

-- 
With respect,
Roman

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