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

You only need to set the min if the drives hide their true sector size, as
Allan mentioned.
 camcontrol identify <drive> is one of the easiest ways to check this.

If the pool reports ashift 12 then zfs correctly detected the drives as 4K
so that part is good

On Tue, 26 Dec 2017 at 20:15, Rodney W. Grimes <
freebsd-rwg_at_pdx.rh.cn85.dnsmgr.net> wrote:

> > 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
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