[gpfsug-discuss] Clarification of mmdiag --iohist output

[IBM Spectrum Scale]

Hi Kevin,

The I/O hist shown by the command mmdiag --iohist actually depends on the 
node on which you are running this command from.
If you are running this on a NSD server node then it will show the time 
taken to complete/serve the read or write I/O operation sent from the 
client node. 
And if you are running this on a client (or non NSD server) node then it 
will show the complete time taken by the read or write I/O operation 
requested by the client node to complete.
So in a nut shell for the NSD server case it is just the latency of the 
I/O done on disk by the server whereas for the NSD client case it also the 
latency of send and receive of I/O request to the NSD server along with 
the latency of I/O done on disk by the NSD server.
I hope this answers your query.


Regards, The Spectrum Scale (GPFS) team

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From:   "Buterbaugh, Kevin L" <Kevin.Buterbaugh at Vanderbilt.Edu>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   02/16/2019 08:18 PM
Subject:        [gpfsug-discuss] Clarification of mmdiag --iohist output
Sent by:        gpfsug-discuss-bounces at spectrumscale.org



Hi All, 

Been reading man pages, docs, and Googling, and haven’t found a definitive 
answer to this question, so I knew exactly where to turn… ;-)

I’m dealing with some slow I/O’s to certain storage arrays in our 
environments … like really, really slow I/O’s … here’s just one example 
from one of my NSD servers of a 10 second I/O:

08:49:34.943186  W        data   30:41615622144   2048 10115.192  srv 
dm-92                  <client IP redacted>

So here’s my question … when mmdiag —iohist tells me that that I/O took 
slightly over 10 seconds, is that:

1.  The time from when the NSD server received the I/O request from the 
client until it shipped the data back onto the wire towards the client?
2.  The time from when the client issued the I/O request until it received 
the data back from the NSD server?
3.  Something else?

I’m thinking it’s #1, but want to confirm.  Which one it is has very 
obvious implications for our troubleshooting steps.  Thanks in advance…

Kevin
—
Kevin Buterbaugh - Senior System Administrator
Vanderbilt University - Advanced Computing Center for Research and 
Education
Kevin.Buterbaugh at vanderbilt.edu - (615)875-9633
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