[CentOS] Re: Read performance on software RAID1?
John R Pierce
pierce at hogranch.com
Thu Apr 12 19:21:52 UTC 2007
WipeOut wrote:
>> AFAIR asks for the data from the raid device, and the md code sends the
>> request to both drives. The drive that replies with data first is the
>> winner.
>> So the system would be reading staggered by that method. Now writes
>> are a
>> different story.
>
> Ok, so in theory then read performance on software RAID1 may be better
> than a single disk but how much better would depend on the hit rate of
> the requests being distributed..
>
> Whats the story on writes?
> You make it sounds like there is something happening there that you
> are not happy about..
writes have to be issued to both drives. assuming the drives are on
seperate channels, or on a shared channel thats fast enough not to block
on two current accesses, then writes should be no slower than a single
drive.
his description of reads is somewhat imprecise. the MD code will send
the request to the drive thats least busy, not to 'both drives'. I
believe it also does some elevator seek optimization (eg: if both drives
are idle, it will go to the one that was last accessed closest to the
new request).
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