[gpfsug-discuss] RAID type for system pool

Wed Sep 5 21:13:17 BST 2018

Hi Kevin,

Why not do single SSD devices and then just use -m DefaultMetadataReplicas
= 3 and -M MaxMetadataReplicas = 3 for your mmcrfs ?  And maybe you can
even get away with -m 2 -M 3.

You will get higher performance overall by having more devices.  You will
get good redundancy with GPFS replicas (just make sure your failure groups
make sense).  Maybe you can split your SSDs across different shelves or
RAID controllers or something.

In any case, if you are creating a new filesystem, you can test all this
out.

Regards,
Alex

On Wed, Sep 5, 2018 at 1:07 PM Uwe Falke <UWEFALKE at de.ibm.com> wrote:

> Hi,
>
> just think that your RAID controller on parity-backed redundancy needs to
> read the full stripe, modify it, and write it back (including parity) -
> the infamous Read-Modify-Write penalty.
> As long as your users don't bulk-create inodes and doo amend some
> metadata, (create a file sometimes, e.g.) The writing of a 4k inode, or
> the update of a 32k dir block causes your controller to read a full block
> (let's say you use 1MiB on MD) and write back the full block plus parity
> (on 4+1p RAID 5 at 1MiB that'll be 1.25MiB. Overhead two orders of
> magnitude above the payload.
> SSDs have become better now  and expensive enterprise SSDs will endure
> quite a lot of full rewrites, but you need to estimate the MD change rate,
> apply the RMW overhead and see where you end WRT lifetime (and
> performance).
>
>
>
>
> Mit freundlichen Grüßen / Kind regards
>
>
> Dr. Uwe Falke
>
> IT Specialist
> High Performance Computing Services / Integrated Technology Services /
> Data Center Services
>
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>
>
> From:   "Buterbaugh, Kevin L" <Kevin.Buterbaugh at Vanderbilt.Edu>
> To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
> Date:   05/09/2018 17:35
> Subject:        [gpfsug-discuss] RAID type for system pool
> Sent by:        gpfsug-discuss-bounces at spectrumscale.org
>
>
>
> Hi All,
>
> We are in the process of finalizing the purchase of some new storage
> arrays (so no sales people who might be monitoring this list need contact
> me) to life-cycle some older hardware.  One of the things we are
> considering is the purchase of some new SSD?s for our ?/home? filesystem
> and I have a question or two related to that.
>
> Currently, the existing home filesystem has it?s metadata on SSD?s ? two
> RAID 1 mirrors and metadata replication set to two.  However, the
> filesystem itself is old enough that it uses 512 byte inodes.  We have
> analyzed our users files and know that if we create a new filesystem with
> 4K inodes that a very significant portion of the files would now have
> their _data_ stored in the inode as well due to the files being 3.5K or
> smaller (currently all data is on spinning HD RAID 1 mirrors).
>
> Of course, if we increase the size of the inodes by a factor of 8 then we
> also need 8 times as much space to store those inodes.  Given that
> Enterprise class SSDs are still very expensive and our budget is not
> unlimited, we?re trying to get the best bang for the buck.
>
> We have always - even back in the day when our metadata was on spinning
> disk and not SSD - used RAID 1 mirrors and metadata replication of two.
> However, we are wondering if it might be possible to switch to RAID 5?
> Specifically, what we are considering doing is buying 8 new SSDs and
> creating two 3+1P RAID 5 LUNs (metadata replication would stay at two).
> That would give us 50% more usable space than if we configured those same
> 8 drives as four RAID 1 mirrors.
>
> Unfortunately, unless I?m misunderstanding something, mean that the RAID
> stripe size and the GPFS block size could not match.  Therefore, even
> though we don?t need the space, would we be much better off to buy 10 SSDs
> and create two 4+1P RAID 5 LUNs?
>
> I?ve searched the mailing list archives and scanned the DeveloperWorks
> wiki and even glanced at the GPFS documentation and haven?t found anything
> that says ?bad idea, Kevin?? ;-)
>
> Expanding on this further ? if we just present those two RAID 5 LUNs to
> GPFS as NSDs then we can only have two NSD servers as primary for them. So
> another thing we?re considering is to take those RAID 5 LUNs and further
> sub-divide them into a total of 8 logical volumes, each of which could be
> a GPFS NSD and therefore would allow us to have each of our 8 NSD servers
> be primary for one of them.  Even worse idea?!?  Good idea?
>
> Anybody have any better ideas???  ;-)
>
> Oh, and currently we?re on GPFS 4.2.3-10, but are also planning on moving
> to GPFS 5.0.1-x before creating the new filesystem.
>
> Thanks much?
>
> ?
> Kevin Buterbaugh - Senior System Administrator
> Vanderbilt University - Advanced Computing Center for Research and
> Education
> Kevin.Buterbaugh at vanderbilt.edu - (615)875-9633
>
>
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