[gpfsug-discuss] RAID type for system pool
Uwe Falke
UWEFALKE at de.ibm.com
Mon Sep 10 00:04:12 BST 2018
Hi, Marc,
I was clearly unaware of that function. If my understanding of
parity-based redundancy is about correct, then that method would only work
with RAID 5, because that is a simple XOR-based hash, but RAID 6, if used,
would not allow that stripped-down RMW. Is that correct?
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: "Marc A Kaplan" <makaplan at us.ibm.com>
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date: 06/09/2018 18:09
Subject: Re: [gpfsug-discuss] RAID type for system pool
Sent by: gpfsug-discuss-bounces at spectrumscale.org
A somewhat smarter RAID controller will "only" need to read the old values
of the single changed segment of data and the corresponding parity
segment, and know the new value of the data block. Then it can compute the
new parity segment value.
Not necessarily the entire stripe. Still 2 reads and 2 writes + access
delay times ( guaranteed more than one full rotation time when on spinning
disks, average something like 1.7x rotation time ).
From: "Uwe Falke" <UWEFALKE at de.ibm.com>
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date: 09/05/2018 04:07 PM
Subject: Re: [gpfsug-discuss] RAID type for system pool
Sent by: gpfsug-discuss-bounces at spectrumscale.org
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
-------------------------------------------------------------------------------------------------------------------------------------------
IBM Deutschland
Rathausstr. 7
09111 Chemnitz
Phone: +49 371 6978 2165
Mobile: +49 175 575 2877
E-Mail: uwefalke at de.ibm.com
-------------------------------------------------------------------------------------------------------------------------------------------
IBM Deutschland Business & Technology Services GmbH / Geschäftsführung:
Thomas Wolter, Sven Schooß
Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht Stuttgart,
HRB 17122
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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