[gpfsug-discuss] sublocks per block in GPFS 5.0

[valleru at cbio.mskcc.org]

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.

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| Block size                    | Subblock size                 |
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| 64 KiB                        | 2 KiB                         |
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| 128 KiB                       | 4 KiB                         |
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| 256 KiB, 512 KiB, 1 MiB, 2    | 8 KiB                         |
| MiB, 4 MiB                    |                               |
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| 8 MiB, 16 MiB                 | 16 KiB                        |
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And you could create more than 1024 sub-blocks per block? and 4k is size of sub-block for 16M?
That is great, since 4K files will go into data pool, and anything less than 4K will go to system (metadata) pool?
Do you think - there would be any performance degradation for reducing the sub-blocks to 4K - 8K, from the default 16K for 16M filesystem?

If we are not loosing any blocks by choosing a bigger block-size (16M) for filesystem, why would we want to choose a smaller block-size for filesystem (4M)?
What advantage would smaller block-size (4M) give, compared to 16M with performance since 16M filesystem could store small files and read small files too at the respective sizes? And Near Line Rotating disks would be happy with bigger block-size than smaller block-size i guess?

Regards,
Lohit

On Mar 30, 2018, 12:45 PM -0400, Marc A Kaplan <makaplan at us.ibm.com>, wrote:
>
> subblock
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