GPFS will issue 4K IOs if the file system is 4K-aligned, which is the
default for 4.1.1+, and may issue requests as small as 512 bytes otherwise.
If a given array presents 4K physical block size to the world, you won't be
able to format a non-4K-aligned file system which puts metadata on that
disk. It is generally believed that 4K IO size produces optimal
performance with flash-based storage in general, because this is the native
physical block size, and 512-byte sector emulation entails doing a
read-modify-write. We don't have the data on how much difference the
sector size actually makes. If in doubt, I would recommend going with 4K
sectors and a 4K-aligned file system.
yuri
From: "Simon Thompson (Research Computing - IT Services)"
<[email protected]>
To: "'[email protected]'"
<[email protected]>,
Date: 05/16/2016 02:48 AM
Subject: Re: [gpfsug-discuss] Flash for metadata
Sent by: [email protected]
Hi Paul,
Thanks, interestingly the FS-900 only does raid5. I get no choice about
that, I do get a sector size choice when I create a lun on it...
I was also wondering how gpfs would do a metadata update. Would it be smart
enough to know the sector size was 512b and only modify that, or would it
always do a read modify write on the whole 4k inode?
Simon
-----Original Message-----
From: Sanchez, Paul [[email protected]]
Sent: Monday, May 16, 2016 10:28 AM GMT Standard Time
To: [email protected]
Subject: Re: [gpfsug-discuss] Flash for metadata
Simon,
I can’t speak specifically to the FS-900, since I generally use Dell
Compellent and NetApp EF arrays for flash today. But I also take care to
ensure that the system pool metadata NSDs use RAID10 (or equivalent) rather
than RAID5/6. For small 4K metadata writes, on a 4K blocksize LUN this
could result in optimal writes. For larger native blocksize LUNs, it
eliminates the parity calculation for read-modify-write operations.
Thanks,
Paul
From: [email protected] [
mailto:[email protected]] On Behalf Of Simon
Thompson (Research Computing - IT Services)
Sent: Monday, May 16, 2016 9:44 AM
To: [email protected]
Subject: [gpfsug-discuss] Flash for metadata
Hi all
I'm currently about to deploy some flash systems (FS-900) to take our
metadata for one of our Spectrum Scale file-systems. We use 4k inodes for
this file-system.
On the FS-900, when creating a volume, we get a choice of 512b or 4k
sectors. Does anyone have a suggestion on this? On a spinning disk RAID
array, I get what a strip size is and we'd use multiples of the stripe
where possible for the block size.
Is the sector size on the FS-900 the smallest chunk that can be written
out? And so would it make sense to use 4k sectors for 4k GPFS inodes right?
Related. In the deployment guide for the FS-900 arrays, if one is using a
file-system (e.g. XFS) on the array the guide talks about aligning the
partitions with with array, when GPFS is used on a storage array, does it
automatically work out the sector alignment?
Thanks
Simon_______________________________________________
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