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

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

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On Thu, 1 Jul 2010 14:40:44 +0800
Shaochun Wang <scwang@ios.ac.cn> wrote:

> 	-bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000
> conv=fdatasync,notrunc Password:
> 	5000+0 records in
> 	5000+0 records out
> 	5242880000 bytes (5.2 GB) copied, 63.497 s, 82.6 MB/s
> 
> 	-bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000
> conv=fdatasync,notrunc 5000+0 records in
> 	5000+0 records out
> 	5242880000 bytes (5.2 GB) copied, 18.1033 s, 290 MB/s
> 	
> I don't know why the second dd becomes 290MB/s and the first 82.6MB/s.

That's because the first time the filesystem had to increase the file's size
5000 times by allocating additional 1 MB, and the second time it was just
writing to an already allocated file. If you see such a big difference here,
run that test 3 or more times, and discard the first run's results.


-- 
With respect,
Roman

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