ZFS raidz and 4k sector disks

[Ivan Voras]

Alexey Tarasov wrote:
> Hello.
> 
> I have reviewed a lot of discussion about new WD 4k sector disks (...EARS).
> I have RAIDZ pool of such disks with very bad performance. Now my GPT ZFS partitions don't start from value dividable by 4 (162).
> Some guys noticed that aligning ZFS partitions according to recommendation wouldn't help at all because RAIDZ uses variable stripe size.

Yes, it doesn't group data in aligned "clusters" - the basic data 
alignment is sector-sized.

> So where is the bottleneck of this configuration: 1) in ZFS which doesn't know about 4k sectorsize? 2) maybe somewhere inside FreeBSD VFS code or disk driver code? 3) somewhere else?

AFAIK (I don't have actual experience with them) current 4k drives 
emulate 512b drives and have a performance penalty in the above 
scenario. Because of this emulation, ZFS doesn't know you have a 4k 
drive. If you can, try disabling this emulation and make it present to 
the operating system as a true 4k drive. Of course, this will make the 
data on it unreadable - you will have to reformat it, and the drive will 
be unbootable.

Good luck and report what you find.