On Mon, May 28, 2012 at 09:23:25AM -0600, Nigel W wrote:
> After a snafu
> last week at $work where a 512 byte pool would not resilver with a 4K
> drive plugged in, it appears that (keep in mind that these are
> consumer drives) Seagate no longer manufactures the 7200.12 series
> drives which has a select-able sector size.  The new 7200.14 series is
> 4k only.  

Does this mean they actually present with 4k sectors externally,
rather than use 4k internally and emulate 512b externally?  If so,
this is a good thing - and good to know.

> WD for the time being appears to still present 512 byte
> sectors in their current lineup. What kind of performance penalty this
> carries I don't know as we have not tested any as of yet.  Presumably
> though, WD is going to stop doing that eventually just like Seagate
> already has.

One hopes so.

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.

For choosing a tradeoff today, I'll take 2 over 1, after experience
with both. 1 bites, seemingly especially with raidz types, but also
with mirrors.  Also because a code change could at least improve the
metadata packing in future.

AFAIK, Hitachi is the only vendor still offering 512-native consumer
drives in the 2&3T sizes.  They cost a little more, so that's another


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