A bunch of disks of different sizes will make it a problem. I wanted to post that idea to the mailing list before, but didn't do so, since it doesn't make too much sense.
Say you have two disks, one 50GB and one 100GB, part of your data can only be ditto'd within the upper 50GB of the larger disk. If the larger one fails, it takes about 25GB of gross data with it, since the ditto blocks couldn't be spread to different disks. More than two disks adds to the chaos. It may reduce the gross data loss of slack ditto blocks (slack as in space that can't be made physically redundant), still isn't too redundant. Note that I say gross data, because it might just happen that a lot of your files may have most of its blocks ditto'd across physical disk, but not all of them. That one specific block in file A that's ditto'd on the same physical disk instead of being spread might just ruin your day. -mg > 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. > > I really cant see this being difficult - but I guess it is dependant on the > zpool remove <vdev> functionality being complete.
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