On 05/28/12 17:13, Daniel Carosone wrote:
There are two problems using ZFS on drives with 4k sectors:

  1) if the drive lies and presents 512-byte sectors, and you don't
     manually force ashift=12, then the emulation can be slow (and
     possibly error prone). There is essentially an internal RMW cycle
     when a 4k sector is partially updated.  We use ZFS to get away
     from the perils of RMW :)

  2) with ashift=12, whther forced manually or automatically because
     the disks present 4k sectors, ZFS is less space-efficient for
     metadata and keeps fewer historical uberblocks.

two, more specific, problems I've run into recently:

1) if you move a disk with an ashift=9 pool on it from a controller/enclosure/.. combo where it claims to have 512 byte sectors to a path where it is detected as having 4k sectors (even if it can cope with 512-byte aligned I/O), the pool will fail to import and appear to be gravely corrupted; the error message you get will make no mention of the sector size change. Move the disk back to the original location and it imports cleanly.

2) if you have a pool with ashift=9 and a disk dies, and the intended replacement is detected as having 4k sectors, it will not be possible to attach the disk as a replacement drive..

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