ZFS raidz2 number of disks

[Michael Laajanen]

Hi,

I have a some questions about ZFS(raidz2) and the number of disks in a vdev.

1 Why is the recommendation for a raidz2 3-9 disk, what are the cons for
having 16 in a pool compared to 8?

2 If having 8 147 GB disk in a vdev under pool00 and later adding 8
300GB under a vdev to pool00, is that a problem or disadvantage compared
to have all 16 in the same size?

3 Is it better to have 8 disk in two vdevs in one pool compared to have
16 in one vdev in one pool?

Hope that someone can clear this out for me

cheers

Michael

[Darren Dunham]

Michael Laajanen <michael_...@yahoo.com> wrote:
> Hi,
>
> I have a some questions about ZFS(raidz2) and the number of disks in a vdev.
>
> 1 Why is the recommendation for a raidz2 3-9 disk, what are the cons for
> having 16 in a pool compared to 8?

Reads potentially have to pull data from all data columns to reconstruct
a filesystem block for verification. For random read workloads,
increasing the number of columns in the raidz does not increase the read
iops. So limiting the column count usually makes sense (with a cost
tradeoff). 16 is valid, but not recommended.

> 2 If having 8 147 GB disk in a vdev under pool00 and later adding 8
> 300GB under a vdev to pool00, is that a problem or disadvantage compared
> to have all 16 in the same size?

Bigger vdevs hold more. So they get a heavier percentage of the
writes. If all vdevs were identical, the load would be spread more
evenly. Otherwise, there's not much difference.

> 3 Is it better to have 8 disk in two vdevs in one pool compared to have
> 16 in one vdev in one pool?

Assuming they are raidz/raidz2 vdevs, the first will have better random
read performance at a capacity cost for the extra parity required.

As another factor, the first will be more robust in the face of
independent disk failures, and will have a smaller time when the system
must reconstruct from failure.

--
Darren

[Michael Laajanen]

Hi,

Thanks for the reply, this atleast confirms my own thoughts about this
and make sense, always some tradeoff money and speed.

I am in middle of upgrade our fileserver from a E250 with a 3300 to a
E450 without the 3300 and need to plan for the new disk bays in the E450.

Since we have a mix of 300(7) and 5(147) in the 3300 I was thinking what
to do with this. Now I feel comfortable with moving to a 8x(mixed
147/300) disk bay and a 8x(pure 300) diskbay in the E450 in two vdevs
and can later grow to only 300 which is nice for budget.

Upgrade to a E450 today might look strange but in a small shop we need
to put the money where it counts(compute servers SunRays) not
fileservers so E450 is not that bad, quit, cheap(cool) stable and spares
on the shelf!

Much thanks
Usernet comp.unix.solaris is one of the highlights about Internet, next
is Google, again thanks :)

/michael