[gpfsug-discuss] GPFS v5: Blocksizes and subblocks

[Dorigo Alvise (PSI)]

Hi Marc,
"Indirect block size" is well explained in this presentation:

http://files.gpfsug.org/presentations/2016/south-bank/D2_P2_A_spectrum_scale_metadata_dark_V2a.pdf

pages 37-41

Cheers,

   Alvise

________________________________
From: gpfsug-discuss-bounces at spectrumscale.org [gpfsug-discuss-bounces at spectrumscale.org] on behalf of Caubet Serrabou Marc (PSI) [marc.caubet at psi.ch]
Sent: Tuesday, March 26, 2019 4:39 PM
To: gpfsug main discussion list
Subject: [gpfsug-discuss] GPFS v5: Blocksizes and subblocks

Hi all,

according to several GPFS presentations as well as according to the man pages:

         Table 1. Block sizes and subblock sizes

+-------------------------------+-------------------------------+
| Block size                    | Subblock size                 |
+-------------------------------+-------------------------------+
| 64 KiB                        | 2 KiB                         |
+-------------------------------+-------------------------------+
| 128 KiB                       | 4 KiB                         |
+-------------------------------+-------------------------------+
| 256 KiB, 512 KiB, 1 MiB, 2    | 8 KiB                         |
| MiB, 4 MiB                    |                               |
+-------------------------------+-------------------------------+
| 8 MiB, 16 MiB                 | 16 KiB                        |
+-------------------------------+-------------------------------+

A block size of 8MiB or 16MiB should contain subblocks of 16KiB.

However, when creating a new filesystem with 16MiB blocksize, looks like is using 128KiB subblocks:

[root at merlindssio01 ~]# mmlsfs merlin
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
.
.
.
 -n                 128                      Estimated number of nodes that will mount file system
 -B                 1048576                  Block size (system pool)
                    16777216                 Block size (other pools)
.
.
.

What am I missing? According to documentation, I expect this to be a fixed value, or it isn't at all?

On the other hand, I don't really understand the concept 'Indirect block size in bytes', can somebody clarify or provide some details about this setting?

Thanks a lot and best regards,
Marc
_________________________________________
Paul Scherrer Institut
High Performance Computing
Marc Caubet Serrabou
Building/Room: WHGA/019A
Forschungsstrasse, 111
5232 Villigen PSI
Switzerland

Telephone: +41 56 310 46 67
E-Mail: marc.caubet at psi.ch
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