[lustre-discuss] varying sequential read performance.

[Mohr Jr, Richard Frank (Rick Mohr)]

John,

I had a couple of thoughts (though not sure if they are directly relevant to your performance issue):

1) Do you know what caching settings are applied on the lustre servers?  This could have an impact on performance, especially if your tests are being run while others are doing IO on the system.

2) It looks like there is a parameter called llite.<fsname>.max_cached_mb that controls how much client side data is cached.  According to the manual, the default value is 3/4 of the host’s RAM (which would be 48GB in your case).  I don’t know why the cache seems to be used unevenly between your 4 OSTs, but it might explain why the cache for some OSTs decrease when others increase.

--
Rick Mohr
Senior HPC System Administrator
National Institute for Computational Sciences
http://www.nics.tennessee.edu


> On Apr 2, 2018, at 8:06 PM, John Bauer <bauerj at iodoctors.com> wrote:
> 
> I am running dd 10 times consecutively to  read a 64GB file ( stripeCount=4 stripeSize=4M ) on a Lustre client(version 2.10.3) that has 64GB of memory.
> The client node was dedicated.
> 
> for pass in 1 2 3 4 5 6 7 8 9 10
> do
>    of=/dev/null if=${file} count=128000 bs=512K
> done
> 
> Instrumentation of the I/O from dd reveals varying performance.  In the plot below, the bottom frame has wall time
> on the X axis, and file position of the dd reads on the Y axis, with a dot plotted at the wall time and starting file position of every read.  
> The slopes of the lines indicate the data transfer rate, which vary from 475MB/s to 1.5GB/s.  The last 2 passes have sharp breaks
> in the performance, one with increasing performance, and one with decreasing performance.
> 
> The top frame indicates the amount of memory used by each of the file's 4 OSCs over the course of the 10 dd runs.  Nothing terribly odd here except that
> one of the OSC's eventually has its entire stripe ( 16GB ) cached and then never gives any up.
> 
> I should mention that the file system has 320 OSTs.  I found LU-6370 which eventually started discussing LRU management issues on systems with high
> numbers of OST's leading to reduced RPC sizes.
> 
> Any explanations for the varying performance?
> Thanks, 
> John
> 
> <johbmffmkkegkbkh.png>
> -- 
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> 
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