freebsd stable - Jul 2009 - glabel metadata protection (WAS: ZFS: drive replacement performance)

>> 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
So what happens if I was to do the following (for the same of example):

gpart create -s GPT /dev/ad1
glabel label -v disk01 /dev/ad1
gpart add -b 1 -s <ENTIREDISK> -t freebsd-zfs /dev/ad1

Does "gpart add" automatically somehow recognize that the last sector
of <ENTIREDISK> contains the glabel and automatically re-adjusts this
command to make the freebsd-zfs partition take "entiredisk minus last
sector" ? I can understand the logic of metadata being protected if I
do a: "gpart add -b 1 -s <ENTIREDISK> -t freebsd-zfs
/dev/label/disk01" since gpart will have to go through the actual
label first, but what actually happens if I issue a gpart directly to
the /dev/device?

- Sincerely,
Dan Naumov

Dan Naumov wrote:
>>> 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? 
I would say in this case you're *not* giving the entire disk to the 
pool, you're giving ZFS a geom that's one sector smaller than the disk. 
  ZFS never sees or can touch the glabel metadata.

> So what happens if I was to do the following (for the same of example):
> 
> gpart create -s GPT /dev/ad1
> glabel label -v disk01 /dev/ad1
> gpart add -b 1 -s <ENTIREDISK> -t freebsd-zfs /dev/ad1
> 
> Does "gpart add" automatically somehow recognize that the last
sector
> of <ENTIREDISK> contains the glabel and automatically re-adjusts this
> command to make the freebsd-zfs partition take "entiredisk minus last
> sector" ? I can understand the logic of metadata being protected if I
> do a: "gpart add -b 1 -s <ENTIREDISK> -t freebsd-zfs
> /dev/label/disk01" since gpart will have to go through the actual
> label first, but what actually happens if I issue a gpart directly to
> the /dev/device?
I'd guess bad stuff would happen here, with a conflict between what gpt 
and glabel would want to do with the end of the disk.

If you wanted to use glabel with a GPT partition, I'd think you'd want
to

   gpart create -s GPT /dev/ad1
   (use "gpart show" to see what space is now available for GPT
    partitions, it won't start at 1 and won't go to the very end of the
    disk)
   gpart add -b 34 -s <REMAININGDISK> -t freebsd-zfs /dev/ad1
   glabel label -v disk01 /dev/ad1p1

   (and then use label/disk01 in a zpool)

	Barry