John R Pierce wrote:
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).
Ok.. That all sounds logical.. Thanks for the feedback.. I figured it
would do something to make use of the two copies of the data but just
wanted to check.. :)
_______________________________________________
CentOS mailing list
CentOS@xxxxxxxxxx
http://lists.centos.org/mailman/listinfo/centos
