On Mon, May 28, 2012 at 6:13 PM, Daniel Carosone <d...@geek.com.au> wrote:
> 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.
Based on the numbers stamped on drive and Seagate support, yes the
7200.14 present 4k sectors and the 7200.12 have a jumper that switches
between 512 and 4k; though I don't know if that means the disk is 4k
or 512 internally.

On Mon, May 28, 2012 at 6:13 PM, Daniel Carosone <d...@geek.com.au> 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.
> 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.
Yes that would suck the performance out and it is something that we
have discussed at $work though so far it seems we have just lucked out
and haven't seen the performance issues as a result of this.

On Mon, May 28, 2012 at 6:13 PM, Daniel Carosone <d...@geek.com.au> wrote:
> 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
> tradeoff.
Hmm. That is interesting to know. At very least another possible
source of 512-byte drives if we need them for replacing drives in
pools that are stuck with ashift=9.
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