Re: [zfs-discuss] RAIDZ2 configuration

Cindy Swearingen Thu, 01 Apr 2010 11:38:41 -0700

Hi Bruno,

I agree that the raidz2 example on this page is weak and I will provide
a better one.


ZFS is very flexible and can be configured many different ways.

If someone new to ZFS wants to take 3 old (but reliable) disks and make
a raidz2 configuration for testing, we would not consider this is a
nonsensical idea. You can then apply what you learn about ZFS space
allocation and redundancy to a new configuration.

If I had only 3 disks, I would create a mirrored configuration of two
disks and keep one as a spare.

Thanks,

Cindy


On 03/31/10 16:02, Bruno Sousa wrote:

Hi Cindy,

This all issue started when i asked opinion in this list in how should i
create zpools. It seems that one of my initial ideas of creating a vdev
with 3 disks in a raidz configuration seems to be a non-sense configuration.
Somewhere along the way i "defended" my initial idea with the fact that
the documentation from Sun has as an example such configuration as seen
here :


*zpool create tank raidz2 c1t0d0 c2t0d0 c3t0d0*  at
http://docs.sun.com/app/docs/doc/819-5461/gcvjg?a=view

So if by concept the idea of having a vdev with 3 disks within a raidz
configuration is a bad one, the oficial Sun documentation should not
have such example. However if people made such example in Sun
documentation, perhaps this all idea is not that bad at all..

Can you provide anything on this subject?

Thanks,
Bruno




On 31-3-2010 23:49, Cindy Swearingen wrote:

Hi Ned,

If you look at the examples on the page that you cite, they start
with single-parity RAIDZ examples and then move to double-parity RAIDZ
example with supporting text, here:

http://docs.sun.com/app/docs/doc/819-5461/gcvjg?a=view

Can you restate the problem with this page?

Thanks,

Cindy


On 03/26/10 05:42, Edward Ned Harvey wrote:

Just because most people are probably too lazy to click the link,
I’ll paste a phrase from that sun.com webpage below:

“Creating a single-parity RAID-Z pool is identical to creating a
mirrored pool, except that the ‘raidz’ or ‘raidz1’ keyword is used
instead of ‘mirror’.”

And

“zpool create tank raidz2 c1t0d0 c2t0d0 c3t0d0”

So … Shame on you, Sun, for doing this to your poor unfortunate
readers.  It would be nice if the page were a wiki, or somehow able
to have feedback submitted…

*From:* zfs-discuss-boun...@opensolaris.org
[mailto:zfs-discuss-boun...@opensolaris.org] *On Behalf Of *Bruno Sousa
*Sent:* Thursday, March 25, 2010 3:28 PM
*To:* Freddie Cash
*Cc:* ZFS filesystem discussion list
*Subject:* Re: [zfs-discuss] RAIDZ2 configuration

Hmm...it might be completely wrong , but the idea of raidz2 vdev with
3 disks came from the reading of
http://docs.sun.com/app/docs/doc/819-5461/gcvjg?a=view .

This particular page has the following example :

*zpool create tank raidz2 c1t0d0 c2t0d0 c3t0d0*

# *zpool status -v tank*

  pool: tank

 state: ONLINE

 scrub: none requested

config:

        NAME          STATE     READ WRITE CKSUM

        tank          ONLINE       0     0     0

          raidz2      ONLINE       0     0     0

            c1t0d0    ONLINE       0     0     0

            c2t0d0    ONLINE       0     0     0

            c3t0d0    ONLINE       0     0     0

So...what am i missing here? Just a bad example in the sun
documentation regarding zfs?

Bruno

On 25-3-2010 20:10, Freddie Cash wrote:

On Thu, Mar 25, 2010 at 11:47 AM, Bruno Sousa <bso...@epinfante.com
<mailto:bso...@epinfante.com>> wrote:

What do you mean by "Using fewer than 4 disks in a raidz2 defeats the
purpose of raidz2, as you will always be in a degraded mode" ? Does
it means that having 2 vdevs with 3 disks it won't be redundant in
the advent of a drive failure?

raidz1 is similar to raid5 in that it is single-parity, and requires
a minimum of 3 drives (2 data + 1 parity)

raidz2 is similar to raid6 in that it is double-parity, and requires
a minimum of 4 drives (2 data + 2 parity)

IOW, a raidz2 vdev made up of 3 drives will always be running in
degraded mode (it's missing a drive).

--

Freddie Cash
fjwc...@gmail.com <mailto:fjwc...@gmail.com>

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Re: [zfs-discuss] RAIDZ2 configuration

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