On Tuesday 22 July 2008 12:18:31 am Steven Schlansker wrote:
Hello -questions,
I have a FreeBSD ZFS storage system working wonderfully with 7.0.
It's set up as three 3-disk RAIDZs -triplets of 500, 400, and 300GB
drives.
I recently purchased three 750GB drives and would like to convert to
using a RAIDZ2. As ZFS has no restriping capabilities yet, I will
have to nuke the zpool from orbit and make a new one. I would like to
verify my methodology against your experience to see if what I wish to
do is reasonable:
I plan to first take 2 of the 750GB drives and make an unreplicated
1.5TB zpool as a temporary storage. Since ZFS doesn't seem to have
the ability to create zpools in degraded mode (with missing drives) I
plan to use iSCSI to create two additional drives (backed by /dev/
zero) to fake having two extra drives, relying on ZFS's RAIDZ2
protection to keep everything running despite the fact that two of the
drives are horribly broken ;)
To make these 500, 400, and 300GB drives useful, I would like to
stitch them together using ccd. I would use it as 500+300 = 800GB and
400+400=800GB
That way, in the end I would have
750 x 3
500 + 300 x3
400 + 400 x 1
400 + 200 + 200 x 1
as the members in my RAIDZ2 group. I understand that this is slightly
less reliable than having real drives for all the members, but I am
not interested in purchasing 5 more 750GB drives. I'll replace the
drives as they fail.
I am wondering if there are any logistical problems. The three parts
I am worried about are:
1) Are there any problems with using an iSCSI /dev/zero drive to fake
drives for creation of a new zpool, with the intent to replace them
later with proper drives?
I don't know about the iSCSI approach but I have successfully created a
degraded zpool using md and a sparse file in place of the missing disk.
Worked like a charm and I was able to transfer everything to the zpool
before nuking the real device (which I had been using for temporary
storage) and replacing the md file with it.
You can create a sparse file using dd:
dd if=/dev/zero of=sparsefile bs=512 seek=(size of the fake device in
512-byte blocks) count=0
Turn it into a device node using mdconfig:
mdconfig -a -t vnode -f sparsefile
Then create your zpool using the /dev/md0 device (unless the mdconfig
operation returns a different node number).
The size of the sparse file should not be bigger than the size of the real
device you plan to replace it with. If using GEOM (which I think you
should, see below), be sure to remember to subtract 512 bytes for each
level of each provider (GEOM modules store their metadata in the last
sector of each provider so that space is unavailable for use). To be on the
safe side you can whack a few KB off.
You can't remove the fake device from a running zpool but the first time you
reboot it will be absent and the zpool will come up degraded.
2) Are there any problems with using CCD under zpool? Should I stripe
or concatenate? Will the startup scripts (either by design or less
likely intelligently) decide to start CCD before zfs? The zpool
should start without me interfering, correct?
I would suggest using gconcat rather than CCD. Since it's a GEOM module (and
you will have remembered to load it via /boot/loader.conf) it will
initialize its devices before ZFS starts. It's also much easier to set up
than CCD. If you are concatenating two devices of the same size you could
consider using gstripe instead, but think about the topology of your drives
and controllers and the likely usage patterns your final setup will create
to decide if that's a good idea.
3) I hear a lot about how you should use whole disks so ZFS can enable
write caching for improved performance. Do I need to do anything
special to let the system know that it's OK to enable the write
cache? And persist across reboots?
Not that I know of. As I understand it ZFS _assumes_ it's working with whole
disks so since it uses its own i/o scheduler performance can be degraded
for anything sharing a physical device with a ZFS slice.
Any other potential pitfalls? Also, I'd like to confirm that there's
no way to do this pure ZFS-like - I read the documentation but it
doesn't seem to have support for nesting vdevs (which would let me do
this without ccd)
You're right, you can't do this with ZFS alone. Good thing FreeBSD is so
versatile. :)
JN
Thanks for any information that you might be able to provide,
Steven Schlansker
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