ZFS question...

Dan Nelson dnelson at allantgroup.com
Thu Apr 10 22:54:48 UTC 2008

In the last episode (Apr 10), Wael Nasreddine said:
> This One Time, at Band Camp, Dan Nelson <dnelson at allantgroup.com> said, On Thu, Apr 10, 2008 at 01:14:02PM -0500:
> > You don't necessarily need ZFS for this; gmirror would work just as
> > well.  You can split your 750GB drive into three
> > partitions/slices/whatevers:
> > 160GB - mirror this with your physical 160GB disk
> > 500GB - mirror this with your physical 500GB disk
> > 90GB - leftover unmirrored, use at your peril
> > ZFS would let you take those two mirrored vdevs and stripe them into a
> > single pool, but then again you could use gstripe or gconcat for that. 
> > The main benefit to ZFS would be if you regularly crash the system;
> > fscking a 750gb UFS filesystem could take a while.
> That's not the desired behaviour actually, what I want is to gain the
> maximum space without the possibility of loosing data, I hear that
> ZFS is excellent at recovering data so I'm trying to figure out the
> perfect installation with these drives and of course while keeping
> the data safe... RAID0 is good for not wasting space at all but then
> again if one drive fails I'll lose everything :(

Thae above config will give you RAID1, not RAID0, since you're
mirroring each small drive onto a part of your large drive.  You'll end
up with 160+500 = 660GB of mirrored storage, with 90gb of unmirrored
space left over.  If you use ZFS, you would do something like this:
Replace /dev/md* with your usb devices, obviously :)

# mdconfig -a -t swap -s 160G
# mdconfig -a -t swap -s 500G
# mdconfig -a -t swap -s 750G
# disklabel -R /dev/md3 /dev/stdin << DONE
 d: 160G * unknown
 e: 500G * unknown
 f: * * unknown
# zpool create usb mirror /dev/md1 /dev/md3d mirror /dev/md2 /dev/md3e
# zpool list usb
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
usb                     655G    112K    655G     0%  ONLINE     -
# zpool status usb
  pool: usb
 state: ONLINE
 scrub: none requested

        NAME        STATE     READ WRITE CKSUM
        usb         ONLINE       0     0     0
          mirror    ONLINE       0     0     0
            md1     ONLINE       0     0     0
            md3d    ONLINE       0     0     0
          mirror    ONLINE       0     0     0
            md2     ONLINE       0     0     0
            md3e    ONLINE       0     0     0

errors: No known data errors
# df -k /usb
Filesystem 1024-blocks Used     Avail Capacity  Mounted on
usb          676085632    0 676085632     0%    /usb

	Dan Nelson
	dnelson at allantgroup.com

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