[gpfsug-discuss] subblock sanity check in 5.0

[Felipe Knop]

Joey,

The subblocks-per-full-block value cannot be specified when the file system
is created, but is rather computed automatically by GPFS. In file systems
with format older than 5.0, the value is fixed at 32. For file systems with
format 5.0.0 or later, the value is computed based on the block size.  See
manpage for mmcrfs, in table where the  -B BlockSize option is explained.
(Table 1. Block sizes and subblock sizes) . Say, for the default (in 5.0+)
4MB block size, the subblock size is 8KB.

The minimum "practical" subblock size is 4KB, to keep 4KB-alignment to
accommodate 4KN devices.

  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:	Joseph Mendoza <jam at ucar.edu>
To:	gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:	06/25/2018 08:59 PM
Subject:	[gpfsug-discuss] subblock sanity check in 5.0
Sent by:	gpfsug-discuss-bounces at spectrumscale.org



Quick question, anyone know why GPFS wouldn't respect the default for
the subblocks-per-full-block parameter when creating a new filesystem?
I'd expect it to be set to 512 for an 8MB block size but my guess is
that also specifying a metadata-block-size is interfering with it (by
being too small).  This was a parameter recommended by the vendor for a
4.2 installation with metadata on dedicated SSDs in the system pool, any
best practices for 5.0?  I'm guessing I'd have to bump it up to at least
4MB to get 512 subblocks for both pools.

fs1 created with:
# mmcrfs fs1 -F fs1_ALL -A no -B 8M -i 4096 -m 2 -M 2 -r 1 -R 2 -j
cluster -n 9000 --metadata-block-size 512K --perfileset-quota
--filesetdf -S relatime -Q yes --inode-limit 20000000:10000000 -T /gpfs/fs1

# mmlsfs fs1
<snipped>

flag                value                    description
------------------- ------------------------
-----------------------------------
 -f                 8192                     Minimum fragment (subblock)
size in bytes (system pool)
                    131072                   Minimum fragment (subblock)
size in bytes (other pools)
 -i                 4096                     Inode size in bytes
 -I                 32768                    Indirect block size in bytes

 -B                 524288                   Block size (system pool)
                    8388608                  Block size (other pools)

 -V                 19.01 (5.0.1.0)          File system version

 --subblocks-per-full-block 64               Number of subblocks per
full block
 -P                 system;DATA              Disk storage pools in file
system


Thanks!
--Joey Mendoza
NCAR
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