ZFS - whole disk or partition or BSD slice?

Steve O'Hara-Smith ateve at sohara.org
Mon Jan 28 11:04:12 UTC 2013

On Mon, 28 Jan 2013 11:40:53 +0200
Volodymyr Kostyrko <c.kworr at gmail.com> wrote:

> 28.01.2013 09:03, Steve O'Hara-Smith:
> > On Sun, 27 Jan 2013 22:05:05 -0800
> > Michael Sierchio <kudzu at tenebras.com> wrote:
> >
> >> On Sun, Jan 27, 2013 at 5:36 PM, Shane Ambler <FreeBSD at shaneware.biz>
> >> wrote:
> >>
> >>> I recall reading that using partitions for zfs on FreeBSD was as good
> >>> as full disks.
> >>
> >> No, it isn't - ZFS can fully utilize disk caches when presented with
> >> whole devices. There are possible reasons to create partitions - one
> >> being that if an unfriendly OS sees the device, it won't try to
> >> initialize it if it sees a partition map.  Another is using a cheap
> >> RAID controller that can't be fully disabled - in which case you
> >> generally need to create a partition that doesn't include the last few
> >> sectors of the disk, where such controllers keep magic data.
> >
> > 	There's one other good reason to use partitions when mirroring.
> > When the time comes to replace a drive in a mirror it is necessary that
> > the new drive be the same size (or larger) than the one it replaces.
> > Given that drives of nominally the same capacity (and even of the same
> > type and brand bought at different times) tend not to be exactly the
> > same size using a partition a little smaller than the whole drive makes
> > it certain that a replacement drive will be big enough to use in the
> > mirror when it arrives.
> There's no need for that as ZFS can use same or bigger partition to 
> mirror existing one. If the second one would be smaller - do some math 
> and cut out some swap space.

	The problem arises when a drive fails, you order a replacement
(go down the shop whatever) and when the new disc arrives and it's a few
blocks smaller than the existing one. Then it cannot be used to mirror the
existing one and you're in for a messy job to get a working mirror up.

Steve O'Hara-Smith <steve at sohara.org>

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