You don't have to do it all at once... ZFS will function fine with 1
large disk and 1 small disk in a mirror, it just means you will only
have the as much space as the smaller disk.
As of things now, if you have multiple vdevs in a pool and they are of
diverse capacities, the striping becomes less effective (? not sure
what the right word here is). Each vdev will be capable of being used
to it's entire capacity.
So suppose in your situation if you have 4 equal disks, configured in
two mirrors. In the future you reach 90% capacity and choose to
upgrade by doubling the size of one vdev. Your pool will use will
strip using the remaining 10% of the original capacity as expected and
then use only the larger vdev from there on. Further in the future if
you then choose to upgrade again by increasing the capacity of the
smaller vdev, the stiping will resume, but there is no way restripe
all of the data evenly across both disks with out copying it off,
removing it and copying it back again.
Overall, I'd would think that if being prepared to upgrade your
entire pool is a concern, that regardless of your zpool configuration
you would want to start saving from day 1 for the upgrade, wait until
the capacity becomes near critical and upgrade the entire pool. This
as opposed to being more reactionary and making a snap decision to buy
what you can afford to bandaid the situation. Afterall, by the time
you get around to upgrading the other vdev's the price/GB will have
dropped even further, assuming you don't replace them with yet even
larger disks than the other vdev.
On 9/27/07, David Dyer-Bennet <dd-b@xxxxxxxx> wrote:
> Sure, that's the same process I have in mind, it's just that you have to
> replace all the disks in the vdev at once to get the new capacity, so I
> felt that sticking to mirrors (meaning only two disks in the vdev) was
> more suitable for my expected future history.
> David Dyer-Bennet, dd-b@xxxxxxxx; http://dd-b.net/
> Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/
> Photos: http://dd-b.net/photography/gallery/
> Dragaera: http://dragaera.info
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