Message-ID: <43838FFB.9060809@tmr.com>
Date: Tue, 22 Nov 2005 16:39:07 -0500
From: Bill Davidsen <davidsen@tmr.com>
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To: Paul Clements <paul.clements@steeleye.com>
CC: Lars Roland <lroland@gmail.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linux RAID M/L <linux-raid@vger.kernel.org>
Subject: Re: Poor Software RAID-0 performance with 2.6.14.2
References: <4ad99e050511211231o97d5d7fw59b44527dc25dcea@mail.gmail.com> <438354B4.10604@tmr.com> <43836214.4010200@steeleye.com>
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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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