ZFS: alignment/boundary for partition type freebsd-zfs

O. Hartmann ohartmann at walstatt.org
Tue Dec 26 17:05:21 UTC 2017

Am Tue, 26 Dec 2017 11:44:29 -0500
Allan Jude <allanjude at freebsd.org> schrieb:

> On 2017-12-26 11:24, O. Hartmann wrote:
> > Running recent CURRENT on most of our lab's boxes, I was in need to replace and
> > restore a ZFS RAIDZ pool. Doing so, I was in need to partition the disks I was about
> > to replace. Well, the drives in question are 4k block size drives with 512b emulation
> > - as most of them today. I've created the only and sole partiton on each 4 TB drive
> > via the command sequence
> > 
> > gpart create -s GPT adaX
> > gpart add -t freebsd-zfs -a 4k -l nameXX adaX
> > 
> > After doing this on all drives I was about to replace, something drove me to check on
> > the net and I found a lot of websites giving "advices", how to prepare large, modern
> > drives for ZFS. I think the GNOP trick is not necessary any more, but many blogs
> > recommend to perform
> > 
> > gpart add -t freebsd-zfs -b 1m -a 4k -l nameXX adaX
> > 
> > to put the partition boundary at the 1 Megabytes boundary. I didn't do that. My
> > partitions all start now at block 40.
> > 
> > My question is: will this have severe performance consequences or is that negligible?
> > 
> > Since most of those websites I found via "zfs freebsd alignement" are from years ago,
> > I'm a bit confused now an consideration performing all this days-taking resilvering
> > process let me loose some more hair as the usual "fallout" ...
> > 
> > Thanks in advance,
> > 
> > Oliver
> >   
> The 1mb alignment is not required. It is just what I do to leave room
> for the other partition types before the ZFS partition.
> However, the replacement for the GNOP hack, is separate. In addition to
> aligning the partitions to 4k, you have to tell ZFS that the drive is 4k:
> sysctl vfs.zfs.min_auto_ashift=12
> (2^12 = 4096)
> Before you create the pool, or add additional vdevs.

I didn't do the sysctl vfs.zfs.min_auto_ashift=12 :-(( when I created the vdev. What is
the consequence for that for the pool? I lived under the impression that this is necessary
for "native 4k" drives.

How can I check what ashift is in effect for a specific vdev?

O. Hartmann

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