I want to setup RAID 1 (mirroring) on a LSI MegaRAID SAS 8708EM2 (1078 chipset) with 4k sector (advanced format) drives. To the best of my knowlegde the controller does not officially support 4k sector drives whereas the newer 9xxxx series controllers (2108 chipset) do. However, I can trick the OS into treating the RAID device as 4k sector drives (FreeBSD geom gnop).

The question is, will 4k sectors on the hardware RAID device (the device presented by the controller to the OS) be mapped one-to-one to 4k sectors on the physical disk?

According to the LSI manual MegaRAID® SAS Software User Guide sec., MegaRAID stores RAID metadata at the end of each physical disk. So the first sector of the RAID device should indeed map to the first sector of the physical drives.

Are there any other factors that affect sector mapping to the disk?

  • Worst case you could test disk performance with up to 8 offsets (0 bytes off, 512 bytes off, 2x512B off, etc etc). Reading your post however I guess that you would be fine with since the RAID meta data is stored at the end of the disk. – Hennes Nov 9 '14 at 11:22
  • Will you be using ZFS? – ewwhite Nov 9 '14 at 14:52
  • @ewwhite Yes, I will probably be using ZFS on some drives, but not on all. If you wanted to hint, that ZFS has built-in RAID / redundancy. I know that... :-) – georg Nov 10 '14 at 13:41

If using ZFS, you can create your pools with ashift=12 if you're dealing with 4k-sector disks.

  • Thank you, that is helpful. ashift sets the minimum IO transfer size for a pool (open-zfs). However gnop on FreeBSD already solves this issue and ashift does not solve the alignment problem. – georg Nov 10 '14 at 17:21

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