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I have 4 2.5" 7.2k drives lying about and I'd like to build some RAID arrays for a bit of VM storage on my KVM lab box.

There are 2x 320GB drives and 2x 500GB drives. I'm looking to create a RAID 10 array using 4x 320GB partitions. Then the remaining 180GB on the 500GB drives I'll build into a RAID 5 partition with another spare drive. I realise that performance will take a hit because half of the drives in the RAID 10 array will be used by another array and using non-matched drives is also not-optimal but this is solely for a lab/test environment.

Initially I thought I would just create 4 matching partitions, create the RAID 10 array, LVM on top and done. But I started looking into all this sector alignment business and now I don't really know where to start.

The 320GB drives report Sector size (logical/physical): 512 bytes / 512 bytes
The 500GB drives report Sector size (logical/physical): 512 bytes / 4096 bytes

So there are the following variables that I would need to take into account: partition start/alignment, RAID stripe size, RAID superblock position (?), LVM PV --dataalignment, then alignment of any file systems in my LVs.

Is anyone able to run through what I'd need to do in this specific case to get optimal alignment/sector/stripe size?

Please try and ignore the fact that the mixed drives mean the whole thing isn't optimal to start with, as I said, this is a test/lab environment.

  • Just align everything to 4k blocks. – Zoredache Feb 14 '14 at 1:01
  • Ok, yes. But things like the RAID superblock being at the beginning of the partition and the LVM meta data which can push things off further... do I need to factor those in? And how do I factor those in? – batfastad Feb 14 '14 at 1:27
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In short, you don't need to worry about it as long as you aren't using really old software. Partitioning tools were patched years ago to align partitions to 1 MiB boundaries so they will be properly aligned. mdadm and lvm also will automatically take care of the alignment.

  • Ok, so I really don't need to bother with askubuntu.com/a/19572 and the LVM alignment here... tytso.livejournal.com/2009/02/20 Is there any definitive documentation to support this? – batfastad Feb 14 '14 at 11:31
  • @benbradley, the official lvm docs I suppose. The raid array exposes its stripe width in sysfs via the optimal_io_size attribute, and lvm uses that as the default --dataalignment value. Oh, and fyi, if you want a better performing raid10 array, use the offset layout with a large ( 8M ) chunk size. You get the sequential read performance of raid0. – psusi Feb 14 '14 at 15:02
  • Ok this is interesting, thanks for the info. 8M does seem large though, most docs I've read on this over the last couple of days only mention chunk sizes of <=1024K – batfastad Feb 16 '14 at 1:29

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