Re: Increasing maxsect of md devices?

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On Wed, 02 Mar 2011 14:26:02 -0800 Bart Kus <me@xxxxxxxx> wrote:

> Hello,
> 
> This seems contradictory:
> 
> jo ~ # blockdev --getiomin /dev/md5
> 524288
> jo ~ # blockdev --getioopt /dev/md5
> 4194304
> jo ~ # blockdev --getmaxsect /dev/md5
> 255
> jo ~ # blockdev --getbsz /dev/md5
> 4096
> jo ~ # blockdev --getss /dev/md5
> 512
> jo ~ #
> 
> Optimal IO size is reported as 4MB (and that is indeed the stripe size), 
> but maximum sectors per request is only 128kB?  How can software do 
> optimal 4MB IOs with the maxsect limit?  Does XFS care about this limit?

md/raid doesn't use 'requests' the any maximum is meaningless.
raid4/5/6 does have a 'stripe cache' which is a vaguely similar thing.  There
can sometimes be value in changing that.

The devices that the array are built from may have a 'maximum sectors per
request', but that probably isn't particularly related to chunk size.

BTW, where do you find "maximum sectors per request in only 128kB" in the
details you quoted - I don't see it.

NeilBrown

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