[gpfsug-discuss] Any guidelines for choosing vdisk size?

[Michael Hennecke]

Ryan,

That is a question with many variables. Some of the factors influencing the decision are:
* Are you sharing data and metadata (the GNR default these days), or are you building separate data and metadata vdisks (the "older" default, and still useful for some use cases)? If you have separate metadata, you need to ensure you leave enough free space in the RGs to account for metadata growth beyond your initial sizing.
* Are you creating vdisks for multiple filesystems on the same building block, or just for one filesystem? 
* How likely is it that space needs to be (re-)allocated between different filesystems?
* How likely is it that new storage will be added to existing filesystems?
* Is there a requirement to include building blocks of different size (different #disks, or different capacity of disks) in the same filesystem?

All of these influence the decision on the "best" vdisk quantity and capacities. If you have a specific configuration question, I'm happy to discuss that offline.
 
In general I advise customers to use at least two equally sized vdisks per RG (4 per building block), so there is more flexibility when capacity needs to be moved around. But many sites that have homogeneous building blocks and don't expect changes in their storage configuration just go with a single vdisk per RG (2 per building block).

Either way, it is a good idea (in environments with more than a single building block) to put aside a small "test" vdisk per RG just for performance testing. So you can do performance validations on each building block individually even if your main filesystem spans all building block. Size of that "test" vdisk should be big enough so you can run a few minutes of sequential IOR against it without filling it up 100%.


Mit freundlichen Grüssen / Best regards,

Michael Hennecke
HPC Chief Technologist - HPC and AI Business Unit  
mailto:mhennecke at lenovo.com
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Geschäftsführung: Colm Gleeson, Christophe Laurent * Sitz der Gesellschaft: Stuttgart * HRB-Nr.: 758298, AG Stuttgart

-----Original Message-----
From: gpfsug-discuss-bounces at spectrumscale.org <gpfsug-discuss-bounces at spectrumscale.org> On Behalf Of Ryan Novosielski
Sent: Monday, 1 July, 2019 07:43
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Subject: [External] [gpfsug-discuss] Any guidelines for choosing vdisk size?

Good morning,

Was wondering if anyone could point me to any tips or provide some regarding choosing vdisk size? My understanding is that too small is a waste of resources in the form of overhead, and that there is an upper limit. but that generally within a pool, you want them to be the same size, so that if you aren’t allocating the entire storage space on the system straight off, you’ll need to choose a reasonable size or be left with unusable space (eg. you will waste space if you went with, say, 200 GB vdisk sizes on a 500 GB array).

Anyone have any tips? Do people just generally allocate the whole thing and have one 2 vdisks (redundancy)? Seems you have some more flexibility if you don’t do that and have to, say, create a storage pool or filesystem from scratch to take advantage of features in a newer FS version or what have you.

Thanks for the help — spent a lot of time looking for this previously, but never asked on the list.
 
--
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