[gpfsug-discuss] GPFS v5: Blocksizes and subblocks

[Stephen Ulmer]

This presentation contains lots of good information about file system structure in general, and GPFS in specific, and I appreciate that and enjoyed reading it.

However, it states outright (both graphically and in text) that storage pools are a feature of the cluster, not of a file system — which I believe to be completely incorrect. For example, it states that there is "only one system pool per cluster", rather than one per file system.

Given that this was written by IBMers and presented at an actual users’ group, can someone please weigh in on this? I’m asking because it represents a fundamental misunderstanding of a very basic GPFS concept, which makes me wonder how authoritative the rest of it is...

-- 
Stephen



> On Mar 26, 2019, at 12:27 PM, Dorigo Alvise (PSI) <alvise.dorigo at psi.ch <mailto:alvise.dorigo at psi.ch>> wrote:
> 
> 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 <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 <mailto:gpfsug-discuss-bounces at spectrumscale.org> [gpfsug-discuss-bounces at spectrumscale.org <mailto:gpfsug-discuss-bounces at spectrumscale.org>] on behalf of Caubet Serrabou Marc (PSI) [marc.caubet at psi.ch <mailto: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               
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