Hi Aaron, 
to how many different files do these tiny I/O requests go?

Mind that the write aggregates the I/O over a limited time (5 secs or so) 
and ***per file***. 
It is for that matter a large difference to write small chunks all to one 
file or to a large number of individual files .
to fill a  1 MiB buffer you need about 13100 chunks of  80Bytes ***per 
file*** within those 5 secs. 

 
Mit freundlichen Grüßen / Kind regards

 
Dr. Uwe Falke
 
IT Specialist
High Performance Computing Services / Integrated Technology Services / 
Data Center Services
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From:   "Knister, Aaron S. (GSFC-606.2)[COMPUTER SCIENCE CORP]" 
<aaron.s.knis...@nasa.gov>
To:     gpfsug main discussion list <gpfsug-discuss@spectrumscale.org>
Date:   10/04/2018 18:09
Subject:        [gpfsug-discuss] Confusing I/O Behavior
Sent by:        gpfsug-discuss-boun...@spectrumscale.org



I hate admitting this but I?ve found something that?s got me stumped. 

We have a user running an MPI job on the system. Each rank opens up 
several output files to which it writes ASCII debug information. The net 
result across several hundred ranks is an absolute smattering of teeny 
tiny I/o requests to te underlying disks which they don?t appreciate. 
Performance plummets. The I/o requests are 30 to 80 bytes in size. What I 
don?t understand is why these write requests aren?t getting batched up 
into larger write requests to the underlying disks. 

If I do something like ?df if=/dev/zero of=foo bs=8k? on a node I see that 
the nasty unaligned 8k io requests are batched up into nice 1M I/o 
requests before they hit the NSD. 

As best I can tell the application isn?t doing any fsync?s and isn?t doing 
direct io to these files. 

Can anyone explain why seemingly very similar io workloads appear to 
result in well formed NSD I/O in one case and awful I/o in another?

Thanks!

-Stumped

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