read-ahead cache on indiv. raid members and entire md device

[Roy Waldspurger <rlists@frameandfocus.com>]

Hi,

I'm looking for advice on tuning the read-ahead cache for an md device...

for example, should I merely set the read-ahead for the md device:

blockdev --setra ### /dev/md2

or should I start touching the individual raid member devices:

blockdev --setra ### /dev/sdc1
blockdev --setra ### /dev/sdd1
blockdev --setra ### /dev/sde1, etc.

I'm not sure about the relation between the two caches.

Also, does the read-ahead cache for the entire md device show-up in 
/proc/ or /sys/block/mdX?  I of course find the read-ahead for the 
individual devices under, for example:

/sys/block/sdc/queue/read_ahead_kb

Many thanks in advance.

Cheers,

-- roy