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

Yes it does know how to figure out based on stripe size

On Tue, 26 Dec 2017 at 20:25, O. Hartmann <ohartmann_at_walstatt.org> wrote:

> Am Tue, 26 Dec 2017 09:31:53 -0800 (PST)
> "Rodney W. Grimes" <freebsd-rwg_at_pdx.rh.CN85.dnsmgr.net> schrieb:
>
> > > 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.
>
>
> I never used any installation scripts so far.
>
> Before I replaced the pool's drives, I tried to search for informations on
> how-to. This
> important tiny fact must have slipped through - or it is very bad
> documented. I didn't
> find a hint in tuning(7), which is the man page I consulted first.
>
> Luckily, as Allan Jude stated, the disk recognition was correct (I guess
> stripesize
> instead of blocksize is taken?).
>
> >
> > > -aLAn
> >
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> >
>
>
>
> --
> O. Hartmann
>
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