> So the question becomes, what are the tradeoffs between running a two-way
> mirror vs. running RAID-Z on two disks?
A two-way mirror would be better -- no parity generation, and you have
the ability to attach/detach for more or less replication. (We could
optimize the RAID-Z code for the two-disk case, but all that would do
is make it into a two-way mirror under the covers.)
> It looks like I can replace one of the disks in the RAID-Z with a bigger one,
> and get more usable space in the pool. I don't believe I can do that in a
> mirror.
With either mirroring or RAID-Z, the available space is determined by
the MIN of all device sizes. If you replace each disk with a larger one,
then when the last replacement is complete, you will indeed have more
space. This is true for mirrors, RAID-Z, or even just single disks.
> I had hoped to be able to add additional disks into an existing RAID-Z
> and expand the available space, but that doesn't seem to be possible
ZFS uses dynamic striping. So rather than growing an existing RAID-Z
group, you just add another one. That is, suppose you create the pool
like this:
zpool create tank raidz disk1 disk2 disk3
To add more space, you'd say:
zpool add tank raidz disk4 disk5 disk6
The down side is that you can't add just one disk -- you have to add them
in small groups. The upside is that by doing it this way, there's no
need for block remapping -- so adding space isn't a violent act.
Jeff
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