ZFS the perfect FS? if only...

Bakul Shah bakul at bitblocks.com
Sat May 12 20:04:29 UTC 2007


> > 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.

Fair enough.

> 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 believe raidz2 means two parity blocks so if you want 50%
parity you'd need raidz4.  But that nit aside, you have a
valid point.

> > 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.

It is not obvious at all that performance would not be a goal
for a freebsd user!

It *is* obvious that you would want more space but not obvious
how to do the conversion from an N disk raidz2 array to N+1
disk raidz2 array *without bringing the whole array down*.
Even you may care about the array being down for hours/days!
Thinking more about this, I believe this can be done without
adding too many complications.  Proof left as an exercise:-)


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