linux-ext4 - Re: ext4 benchmark questions

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Message-ID: <m2h1f4ef0971004221511wa0ea3d3fteb90ad1c38c9511d@mail.gmail.com>
Date:	Thu, 22 Apr 2010 17:11:20 -0500
From:	Steve Brown <sbrown25@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: ext4 benchmark questions

>> 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.

So should I see any performance change by using the noatime mount option at all?

>> 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...

hmmm... this would seem to conflict with the docs in the kernel, especially:

"Write barriers enforce proper on-disk ordering
of journal commits, making volatile disk write caches
safe to use, at some performance penalty.  If
your disks are battery-backed in one way or another,
disabling barriers may safely improve performance."

>> 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.

I'm not worried about powerloss.  The kernel docs seem to imply that
data=[journaled,ordered] come with a performance hit.  My results
would indicate otherwise.  Should I be seeing this kinda of
performance difference?

>> 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...

Steve
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