ZFS the perfect FS? if only...

Peter Schuller peter.schuller at infidyne.com
Sat May 12 07:29:54 UTC 2007

> You can do something like this:
> zpool create foo raidz2 dev0 dev1 dev2 dev3
> zpool add    foo raidz2 dev4 dev5 dev6 dev7
> The new devices are in a different raidz2 group but but *all
> of the space* will be used for any filesystem on this pool --
> isn't this good enough and if not, why?

Isn't it obvious?

You waste less diskspace. In the above setup you are only using
half your diskspace and depending on what you need it for you might
aswell have been using a mirror...

Often having two parity drives can be more important than the ratio of
parity vs. non-parity. That is, just because one chose to start with a 4
disk raidz2, it does not mean that moving to an 8 disk raidz2 is not
consistent with the redundancy goals/requirements chosen initially.

Also, if you DO want 50% parity, a single raidz2 with 8 drives with 50%
parity is more resilient to failures than 2x4 raidz2 since any 4 drives
can fail whereas in the latter case any 2 drives can fail, or up 4
drives if they happen to be the right drives.

(I am aware of the "approximately 7 drives" maximum for a raidz/raidz2,
which does not really come into play in this case.)

> Not worth it

Most definitely worth it in many situations where performance is just
not the goal. So what if it takes a week to perform the operation, as
long as the array is not degraded during this timewindow.

/ Peter Schuller

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