Couple of related questions.

Background: I have a stock of drives lying about that I want to use in a HP Proliant Microserver which has a 4 slot SAS cage with FreeNas. I don't want to spend any more on parts.

1) With Zfs and RAIDZ, how bad would it be to mix 2 x 1TB Seagate Barracudas with 2 x 1TB WD Green drives. The latter Green drives are I believe only 5,400 RPM, but I can't find that info for sure.

2) Which would be better, the above mis-match in a RAIDZ or 2 x 3TB Seagate Barracuda in a mirror. Considering both performance and data security

3) I have a spare 250GB drive and a spare SATA slot (for optical drive, not sure on speed) as well, would that be a performance gain to use that for the ZIL and L2ARC (over just using the main drives).

Thanks in advance.

  1. If your raid-z VDEV has mixed spindle speeds speeds performance will be like the slowest disks (IO will arrive sooner from the 7200rpm disks, but will wait for data from the 5400rpm before the block is reconstructed.) Your greens are somewhere in the 5400-5900rpm range.

  2. (A) Data security: mirror wins. You'd survive a single disk failing and if you were lucky (33% chance) you could survive a second disk too. Rebuild times are faster with mirrors too. (B) Performance: mirror wins. Double the write performance, better read performance since simultaneous reads are serviced by both halves of the mirror. (C) Capacity (not asked): with mismatched disk sizes raid-z will be limited to the smallest disk (2x1TB + 2x3TB acts just like 4x1TB, so 3TB usable). With mirrors you get all that capacity (2x1TB + 2x3TB = 4TB usable).

  3. A random extra hard disk as L2ARC or ZIL likely won't yield measurable performance gains for most workloads. Maybe as ZIL if the disk was faster than pool media (10k vs 7200rpm). If used as L2Arc maybe a few extra pool IOPS could be eeked under load from any L2Arc cache hits, but I can't imagine it's worth it using an extra old hard drive.

Don't forget to use ashift=12 at pool creation time if your disks have 4K sectors (WD 1TB Green 'EARS' Drives do).

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  • Thanks for the advice, I half expected as much. I didn't want to compare capacity as both options would net 3TB usable. – Martyn Oct 18 '12 at 8:39
  • +1 Also on #3, it would probably be slower to use the 250GB platter drive as it would have a slower total throughput than the array of disks. In practice a L2Arc or ZIL should never be anything less than a SSD, PCIe Flash cards work well too. – Chris S Oct 18 '12 at 20:02

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