Using GTP and glabel for ZFS arrays

[Paul Mather]

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

> I hope my terminology is correct....
> 
> 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.
> 
> Why use glabel?
> 
> * 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?
> 
> * 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.
> 
> * 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.