[gpfsug-discuss] segment size and sub-block size

[Aaron Knister]

Hi Brian,

We use a 128KB segment size on our DDNs and a 1MB block size and it 
works quite well for us (throughput in the 10's of gigabytes per second).

IIRC the sub block (blockSize/32) is the smallest unit of allocatable 
disk space. If that's not tuned well to your workload you can end up 
with a lot of wasted space on the filesystem.

In option #1, the smallest unit of allocatable space is 256KB. If you 
have millions of files that are say 8K in size you can do the math on 
lost space.

In option #2, if you're using the same 1MB segment size from the option 
1 scenario it gets even worse.

Hope that helps. This might also help 
(https://www.ibm.com/support/knowledgecenter/STXKQY_4.2.0/com.ibm.spectrum.scale.v4r2.ins.doc/bl1ins_frags.htm).

-Aaron

On 7/16/16 9:04 PM, Brian Marshall wrote:
> All,
>
> When picking blockSize and segmentSize on RAID6 8+2 LUNs, I have see 2
> optimal theories.
>
>
> 1) Make blockSize = # Data Disks * segmentSize
> e.g. in the RAID6 8+2 case,  8 MB blockSize = 8 * 1 MB segmentSize
>
> This makes sense to me  as every GPFS block write is a full stripe write
>
> 2) Make blockSize = 32 (number sub blocks) * segmentSize; also make sure
> the blockSize is a multiple of #data disks * segmentSize
>
> I don't know enough about GPFS to know how subblocks interact and what
> tradeoffs this makes.
>
> Can someone explain (or point to a doc) about sub block mechanics and
> when to optimize for that?
>
> Thank you,
> Brian Marshall
>
>
>
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-- 
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776