Hi Kevin, as you already pointed out, having a RAID stripe size (or a multiple of it) not matching GPFS blocksize, is a bad idea. Every write would cause a read-modify-write operation to keep the parity. So for data LUNs RAID5 with 4+P or 8+P is fully ok. For metadata, if you are keen on performance, I would stay with RAID1, or even RAID0, so you aren’t affected by possible RAID rebuild performance drops. Regards, Achim
> Am 05.09.2018 um 17:35 schrieb Buterbaugh, Kevin L > <kevin.buterba...@vanderbilt.edu>: > > 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.buterba...@vanderbilt.edu - (615)875-9633 > > >
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