[Lustre-discuss] Performance Drop creating big files

[Kumaran Rajaram]

Roger,

I would suspect the Lustre client side caching influencing your write
performance. 10GB is not adequate, try atleast 4x of the RAM size for
file-sizes. Try doing the same tests with O_DIRECT flag as it'd truly
measure your disk I/O performance bypassing FS + buffer cache.

HTH,
-Kums

On Fri, 2008-05-30 at 16:25 -0400, Roger Spellman wrote:
> I am seeing the following odd behavior.  I have several OSSes, each
> with a 7T RAID 5.  
> 
>  
> 
> If I use a single client to create a single 1T file which is striped
> to a single OST, the performance starts off at about 400 MB/s (which
> is typical for my HW), then gradually decreases, until it reaches 250
> MB/s.  I’ve seen this with both IOZone and dd.
> 
> 
> As an experiment, I wrote a script that creates two hundred 10G files,
> without removing them.  Again, the performance starts off at 400 MB/s.
> But, the performance stays nearly the same throughout the test.
> 
>  
> 
> The only difference between these tests is that in the second case,
> there are lots of opens & closes, and in the first case, just a single
> open and close.
> 
>  
> 
> Can anyone explain what is happening here, and how to possible fix it?
> 
>  
> 
> Thanks.
> 
>  
> 
> Roger Spellman
> 
> Sr. Staff Engineer
> 
> Terascala, Inc.
> 
>  
> 
> 
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