linux-ext4 - Re: ext4 benchmark questions

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Date:	Thu, 22 Apr 2010 16:52:10 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Steve Brown <sbrown25@...il.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: ext4 benchmark questions

Steve Brown wrote:
...

> I'll start with the craziest one: noatime.  Everything I have read
> says that the noatime option should increase both read and write
> performance.  My results are finding that write speeds are comparable
> with or without this option, but read speeds are significantly faster
> *without* the noatime option.  For example, a 16GB file reads about
> 210MB/s with noatime but reads closer to 250MB/s without the noatime
> option.

the kernel uses "relatime" now by default, which gives you most of the
benefit already.

> Next is the write barrier.  I'm an in a fully battery-backed
> environment, so I'm not worried about disabling it.  From my testing,
> setting barrier=0 will improve write performance on large files
> (>10GB), but hurts performance on smaller files (<10GB).  Read
> performance is effected similarly.  Is this to be expected with files
> of this size?

not expected by me; barriers == drive write cache flushes, which I
would never expect to speed things up...

> Next is the data option.  I am seeing a significant increase in read
> performance when using data=ordered vs data=writeback.  Reading is as
> much as 20% faster when using data=ordered.  The difference in write
> performance is almost none with this option.

data=writeback is not safe for data integrity; unless you can handle
scrambled files post-crash/powerloss, don't use it.

> Finally is the commit option.  I did my testing mounting with commit=5
> and commit=90.  While my read performance increased with commit=90, my
> write performance improved by as much as 30% or more with commit=5.

not sure offhand what to make of decreased write performance with a longer
commit time...

-Eric
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