Paul Hedderly wrote:
Strikes me that at the moment Sun/ZFS team is missing a great opportunity.
Imagine Joe bloggs has a historical machine with Just Any Old Bunch Of Discs...
(it's not me, no really).
He doesn't want to have to think too hard about pairing them up in mirrors or
in raids - and sometimes they die or are just too small so need to get swapped
out - or maybe they are iSCSI/AoE targets that might disappear (say the 'spare
space' on a thousand desktop PC's...)
What Joe really wants to say to ZFS is: "Here is a bunch of discs. Use them any way
you like - but I'm setting 'copies=2' or 'stripes=5' and 'parity=2' so you just go
allocating space on any of these discs trying to make sure I always have resilliance at
the data level."
Now I can do that at the moment - well the copies/ditto kind anyway - but if I
lose or remove one of the discs, zfs will not start the zpool. [i]That
sucks!!![/i]
Because... if one disc has gone from a bunch of 10 or so, and I have all my
data and metadata using dittos, then the data that was on that disc is
replicated on the others - so losing one disc is not a problem (unless there
wasn't space to store all the copies on the other discs, I know) but zfs should
be able to start that zpool and give me the option to reditto the data that has
lost copies on the dead/removed disc.
So I get nice flexible "mirroring" by just throwing a JAOBOD at zfs and it does
all the hard work.
There is a fundamental difference between mirroring and ditto blocks. The
allocation
of data onto devices is guaranteed to be on both sides of a mirror. The
allocation of
ditto blocks depends on the available space -- two or more copies of the data
could be
placed on the same device. If that device is lost, then data is lost.
I expect the more common use of ditto blocks will be for single-disk systems
such as
laptops. This is a good thing for laptops.
I really cant see this being difficult - but I guess it is dependant on the zpool
remove <vdev> functionality being complete.
There is another issue here. As you note, figuring out the optimal placement
of data
for a given collection of devices can be more work than we'd like. We can
solve this
problem using proven mathematical techniques. If such a wizard was available,
would
you still want to go down the path of possible data loss due to a single device
failure?
-- richard
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