Re: ZFS: alignment/boundary for partition type freebsd-zfs

On Tue, Dec 26, 2017 at 10:04 AM, O. Hartmann <ohartmann_at_walstatt.org>
wrote:

> 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?
>

It's only necessary if your drive stupidly fails to report its physical
sector size correctly, and no other FreeBSD developer has already written a
quirk for that drive.  Do "zdb -l /dev/adaXXXpY" for any one of the
partitions in the ZFS raid group in question.  It should print either
"ashift: 12" or "ashift: 9".

-Alan