Re: Write-intent bitmap decreases or increase performance of RAID5?

[Roman Mamedov <roman@rm.pp.ru>]

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On Wed, 30 Jun 2010 04:11:54 +0800
Shaochun Wang <scwang@ios.ac.cn> wrote:

> From what I read from the internet, write-intent bitmap decreases raid
> performance and provides more stability. But in my situation, it also
> increases raid performance dramatically!
> 
> My RAID5 write speed without write-intent bitmap is 90MB/s, and is
> increased to 200MB/s after enabling internal write-intent bitmap. I
> use ext4 filesystem on this raid array.

How do you measure that speed exactly? The proper (and also quite simple) way
to do it is to run the following test several times (assuming that current dir
is some directory on the array):

  dd if=/dev/zero of=zerofile bs=1M count=2048 conv=fdatasync,notrunc

Also, I see you run an 8-drive RAID5 on 1.5TB drives, so make sure you are
aware of the issues described in:
  http://hardware.slashdot.org/article.pl?sid=08/10/21/2126252
  http://louwrentius.blogspot.com/2008/08/why-raid-1-and-5-will-kill-you-some-day.html

-- 
With respect,
Roman

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