ZFS: drive replacement performance

[Brooks Davis]

On Wed, Jul 08, 2009 at 01:40:02AM +0300, Dan Naumov wrote:
> On Wed, Jul 8, 2009 at 1:32 AM, Freddie Cash<fjwcash at gmail.com> wrote:
> > On Tue, Jul 7, 2009 at 3:26 PM, Mahlon E. Smith <mahlon at martini.nu> wrote:
> >
> >> On Tue, Jul 07, 2009, Freddie Cash wrote:
> >> >
> >> > This is why we've started using glabel(8) to label our drives, and then
> >> add
> >> > the labels to the pool:
> >> > ? # zpool create store raidz1 label/disk01 label/disk02 label/disk03
> >> >
> >> > That way, it does matter where the kernel detects the drives or what the
> >> > physical device node is called, GEOM picks up the label, and ZFS uses the
> >> > label.
> >>
> >> Ah, slick. ?I'll definitely be doing that moving forward. ?Wonder if I
> >> could do it piecemeal now via a shell game, labeling and replacing each
> >> individual drive? ?Will put that on my "try it" list.
> 
> Not to derail this discussion, but can anyone explain if the actual
> glabel metadata is protected in any way? If I use glabel to label a
> disk and then create a pool using /dev/label/disklabel, won't ZFS
> eventually overwrite the glabel metadata in the last sector since the
> disk in it's entirety is given to the pool? Or is every filesystem
> used by FreeBSD (ufs, zfs, etc) hardcoded to ignore the last few
> sectors of any disk and/or partition and not write data to it to avoid
> such issues?

Disks labeled with glabel lose their last sector to the label.  It is not
accessible by ZFS.  Disks with bsdlabel partition tables are at risk due to
the brain dead decision to allow partitions to overlap the first sector,
but modern designs like glabel avoid this mistake.

-- Brooks
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