I've heard it highly recommended (and have been *really* glad at times
to have it) to have at least 2 replicas of metadata to help maintain fs
consistency in the event of fs issues or hardware bugs (e.g. a torn write).
-Aaron
On 9/5/18 1:37 PM, Frederick Stock wrote:
Another option for saving space is to not keep 2 copies of the metadata
within GPFS. The SSDs are mirrored so you have two copies though very
likely they share a possible single point of failure and that could be a
deal breaker. I have my doubts that RAID5 will perform well for the
reasons Marc described but worth testing to see how it does perform. If
you do test I presume you would also run equivalent tests with a RAID1
(mirrored) configuration.
Regarding your point about making multiple volumes that would become
GPFS NSDs for metadata. It has been my experience that for traditional
RAID systems it is better to have many small metadata LUNs (more IO
paths) then a few large metadata LUNs. This becomes less of an issue
with ESS, i.e. there you can have a few metadata NSDs yet still get very
good performance.
Fred
__________________________________________________
Fred Stock | IBM Pittsburgh Lab | 720-430-8821
[email protected]
From: "Marc A Kaplan" <[email protected]>
To: gpfsug main discussion list <[email protected]>
Date: 09/05/2018 01:22 PM
Subject: Re: [gpfsug-discuss] RAID type for system pool
Sent by: [email protected]
------------------------------------------------------------------------
It's good to try to reason and think this out... But there's a good
likelihood that we don't understand ALL the details, some of which may
negatively impact performance - so no matter what scheme you come up
with - test, test, and re-test before deploying and depending on it in
production.
Having said that, I'm pretty sure that old "spinning" RAID 5
implementations had horrible performance for GPFS metadata/system pool.
Why? Among other things, the large stripe size vs the almost random
small writes directed to system pool.
That random-small-writes pattern won't change when we go to SSD RAID 5 -
so you'd have to see if the SSD implementation is somehow smarter than
an old fashioned RAID 5 implementation which I believe requires several
physical reads and writes, for each "small" logical write.
(Top decent google result I found quickly
_http://rickardnobel.se/raid-5-write-penalty/_But you will probably want
to do more research!)
Consider GPFS small write performance for: inode updates, log writes,
small files (possibly in inode), directory updates, allocation map
updates, index of indirect blocks.
From: "Buterbaugh, Kevin L" <[email protected]>
To: gpfsug main discussion list <[email protected]>
Date: 09/05/2018 11:36 AM
Subject: [gpfsug-discuss] RAID type for system pool
Sent by: [email protected]
------------------------------------------------------------------------
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_
[email protected]_
<mailto:[email protected]>- (615)875-9633
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--
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776
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