Re: Poor Software RAID-0 performance with 2.6.14.2

[Bill Davidsen <davidsen@tmr.com>]

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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