[gpfsug-discuss] sublocks per block in GPFS 5.0

[Felipe Knop]

Folks,

Also quoting a previous post:

         Thanks Mark,

         I did not know, we could explicitly mention sub-block size when
         creating File system. It is no-where mentioned in the “man
         mmcrfs”.
         Is this a new GPFS 5.0 feature?

         Also, i see from the “man mmcrfs” that the default sub-block size
         for 8M and 16M is 16K.


Specifying the number of subblocks per block or the subblock size in mmcrfs
is not currently supported.  The subblock size is automatically chosen
based on the block size, as described in 'Table 1. Block sizes and subblock
sizes'  in 'man mmcrfs'.

  Felipe

----
Felipe Knop                                     knop at us.ibm.com
GPFS Development and Security
IBM Systems
IBM Building 008
2455 South Rd, Poughkeepsie, NY 12601
(845) 433-9314  T/L 293-9314





From:	"Marc A Kaplan" <makaplan at us.ibm.com>
To:	gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:	03/30/2018 02:48 PM
Subject:	Re: [gpfsug-discuss] sublocks per block in GPFS 5.0
Sent by:	gpfsug-discuss-bounces at spectrumscale.org



Look at my example, again, closely.  I chose the blocksize as 16M and
subblock size as 4K and the inodesize as 1K....
Developer works is a good resource, but articles you read there may be
incomplete or contain mistakes.

The official IBM Spectrum Scale cmd and admin guide documents, are
"trustworthy" but may not be perfect in all respects.  "Trust but Verify"
and YMMV. ;-)

As for why/how to choose "good sizes", that depends what objectives you
want to achieve, and "optimal" may depend on what hardware you are running.
Run your own trials and/or ask performance experts.  There are usually
"tradeoffs" and OTOH when you get down to it, some choices may not be
all-that-important in actual deployment and usage.  That's why we have
defaults values - try those first and leave the details and tweaking aside
until you have good reason     ;-)
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