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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