Re: Using GTP and glabel for ZFS arrays

On Jul 21, 2010, at 11:05 PM, Dan Langille wrote:

> I hope my terminology is correct....
>=20
> I have a ZFS array which uses raw devices.  I'd rather it use glabel =
and supply the GEOM devices to ZFS instead.  In addition, I'll also =
partition the HDD to avoid using the entire HDD: leave a little bit of =
space at the start and end.
>=20
> Why use glabel?
>=20
> * So ZFS can find and use the correct HDD should the HDD device ever
>   get renumbered for whatever reason.  e.g. /dev/da0 becomes /dev/da6
>   when you move it to another controller.

I have created ZFS pools using this strategy.  However, about a year ago =
I still fell foul of the drive shuffling problem, when GEOM labels =
appeared not to be detected properly:

	=
http://lists.freebsd.org/pipermail/freebsd-geom/2009-July/003654.html

This was using RELENG_7, and the problem was provoked by external USB =
drives.

The same issue might not occur with FreeBSD 8.x, but I thought I'd point =
out my experience as a possible warning about using glabel.

Nowadays, I use GPT labels ("gpart ... -l somelabel", referenced via =
/dev/gpt/somelabel).

> Why use partitions?
>=20
> * Primarily: two HDD of a given size, say 2TB, do not always provide
>   the same amount of available space.  If you use a slightly smaller
>   partition instead of the entire physical HDD, you're much more
>   likely to have a happier experience when it comes time to replace an
>   HDD.
>=20
> * There seems to be a consensus amongst some that leaving the start =
and
>   and of your HDD empty.  Give the rest to ZFS.

You should also try and accommodate 4K sector size drives these days.  =
Apparently, the performance boosts from hitting 4K-aligned sectors can =
be very good.

Cheers,

Paul.=
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