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
tradeoff.
--
Dan.
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