Messages in this thread | | | Date | Tue, 22 Nov 2005 16:39:07 -0500 | From | Bill Davidsen <> | Subject | Re: Poor Software RAID-0 performance with 2.6.14.2 |
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Paul Clements wrote: > Bill Davidsen wrote: > >> One of the advantages of mirroring is that if there is heavy read load >> when one drive is busy there is another copy of the data on the other >> drive(s). But doing 1MB reads on the mirrored device did not show that >> the kernel took advantage of this in any way. In fact, it looks as if >> all the reads are going to the first device, even with multiple >> processes running. Does the md code now set "write-mostly" by default >> and only go to the redundant drives if the first fails? > > > No, it doesn't use write-mostly by default. The way raid1 read balancing > works (in recent kernels) is this: > > - sequential reads continue to go to the first disk > > - for non-sequential reads, the code tries to pick the disk whose head > is "closest" to the sector that needs to be read > > So even if the reads aren't exactly sequential, you probably still end > up reading from the first disk most of the time. I imagine with a more > random read pattern you'd see the second disk getting used.
Thanks for the clarification. I think the current method is best for most cases, I have to think about how large a file you would need to have any saving in transfer time given that you have to consider the slowest seek, drives doing other things on a busy system, etc.
-- -bill davidsen (davidsen@tmr.com) "The secret to procrastination is to put things off until the last possible moment - but no longer" -me
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