Re: paralellism of device use in md

[Mario 'BitKoenig' Holbe <Mario.Holbe@TU-Ilmenau.DE>]

Tim Moore <linux-raid@nsr500.net> wrote:
> Andy Smith wrote:
>> Are reads from a 2 device RAID-1 twice as fast as from a single
> md14 : active raid0 sdb13[1] sda13[0]
> md13 : active raid1 sdb12[1] sda12[0]
>
> /dev/md14:
>   Timing buffered disk reads:  272 MB in  3.01 seconds =  90.37 MB/sec
> /dev/md13:
>   Timing buffered disk reads:  164 MB in  3.00 seconds =  54.67 MB/sec

And this is exactly the strange thing which I'm also experiencing and
which was asked a lot of times on this list already, IIRC.

Why is the single-stream read-performance of a RAID1 so much worse than
the read-performance of a RAID0. A RAID1 should easily be able to gain
(or perhaps even advance, since it's not bound to chunk borders) the
read-performance of a RAID0.

As far as I can see, RAID1 only does that in case of lots of parallel
scheduled read-requests. Would it probably make sense to split one
single read over all mirrors that are currently idle?


regards
   Mario
-- 
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and do a better job on the wings."            -- Stanley Kubrick