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".

And more than likely if you used the bsdinstall from one of
the distributions to setup the system you created the ZFS
pool from it has the sysctl in /boot/loader.conf as the
default for all? recent?  bsdinstall's is that the 4k default
is used and the sysctl gets written to /boot/loader.conf
at install time so from then on all pools you create shall
also be 4k.   You have to change a default during the
system install to change this to 512.
 
> -aLAn

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-- 
Rod Grimes                                                 rgrimes_at_freebsd.org