followup storage question

Graham Allan allan at
Fri Sep 11 15:27:47 UTC 2015

On 9/11/2015 7:59 AM, William A. Mahaffey III wrote:
> The Wiki page
> illustrates using gnop to enforce 4K alignment of gpt partitions for
> subsequent use by ZFS. However the gpart commands also use the '-a 4k'
> arguments, aligning partitions on 4k boundaries as I understand things.
> Is the gnop command also necessary ? TIA & have a nice weekend.

My experience is I only ever install on smaller devices so never really 
worry about 4k alignment at that time, only for making large storage 
pools later. With that mind mind here is my "easy" answer for 9.3 and 
10.x. The installer "auto ZFS" has a 4k alignment option which I would 
assume works as advertised (having said that it would be simple to do a 
basic install and then examine that to confirm). For making subsequent 
pools, I simply set vfs.zfs.min_auto_ashift=12 is /etc/sysctl.conf, 
after which you will get 4k block alignment without messing with gnop. 
On 9.1 you had to do the gnop tricks but 9.3 supports the auto_ashift 

On 9.3+, "zpool status" also *tells you* if you got the block alignment 
wrong, which is a big help over 9.1 where it could lurk undetected!


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