ZFS on labelled partitions (was: Re: LSI SAS2008 mps driver preferred firmware version)

[Freddie Cash]

On Tue, Nov 17, 2015 at 12:08 AM, Patrick M. Hausen <hausen at punkt.de> wrote:

> Hi, all,
>
> > Am 16.11.2015 um 22:19 schrieb Freddie Cash <fjwcash at gmail.com>:
> >
> > ​You label the disks as they are added to the system the first time.
> That
> > way, you always know where each disk is located, and you only deal with
> the
> > labels.
>
> we do the same for obvious reasons. But I always wonder about the possible
> downsides, because ZFS documentation explicitly states:
>
>         ZFS operates on raw devices, so it is possible to create a storage
> pool comprised of logical
>         volumes, either software or hardware. This configuration is not
> recommended, as ZFS works
>         best when it uses raw physical devices. Using logical volumes
> might sacrifice performance,
>         reliability, or both, and should be avoided.
>
> (from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)
>
> Can anyone shed some lght on why not using raw devices might sacrifice
> performance or reliability? Or is this just outdated folklore?
>

​On Solaris, using raw devices allows ZFS to enable the caches on the disks
themselves, while using any kind of partitioning on the disk forces the
caches to be disabled.

This is not an issue on FreeBSD due to the way GEOM works.  Caches on disks
are enabled regardless of how the disk is accessed (raw, dd-partitioned,
MBR-partitioned, GPT-partitioned, gnop, geli, whatever).

This is a common misconception and FAQ with ZFS on FreeBSD and one reason
to not take any Sun/Oracle documentation at face value, as it doesn't
always apply to FreeBSD.

There were several posts from pjd@ about this back in the 7.x days when ZFS
was first imported to FreeBSD.

-- 
Freddie Cash
fjwcash at gmail.com