Sunil wrote:
If you like, you can later add a fifth drive
relatively easily by
replacing one of the slices with a whole drive.
how does this affect my available storage if I were to replace both of those
sparse 500GB files with a real 1TB drive? Will it be same? Or will I have
expanded my storage? If I understand correctly, I would need to replace other 3
drives with 1TB as well to expand beyond 3X500GB.
So, in essence I can go from 3x500GB to 3X1000GB in-place with this scheme in
future if I have the money to upgrade all the drives to 1TB, WITHOUT needing
any movement of data to temp? Please say yes!....:-)
It should work to replace devices the way you describe. The only time
you need some temp storage space is if you want to change the
arrangement of devices that make up the pool, e.g. to go from
striped-mirrors to RAIDZ2, or RAIDZ1 to RAIDZ2, or some other
combination. If you just want to replace devices with identical or
larger sized devices you don't need to move the data anywhere.
The capacity will expand to the lowest common denominator. In some
OpenSolaris builds I believe this happened automatically when all member
devices had been upgraded. At some point in later builds I think it was
changed to require manual intervention to prevent problems (like the
pool suddenly growing to fill all the new big drives when the admin
really wanted the unused space to stay unused..say for partition/slice
based short stroking, or when smaller drives were being kept around as
spares. If ZFS had the ability to shrink and use smaller devices this
would not have been as big of a problem.
As I understand it from the documentation, replacement can happen two
ways. First, you can connect the replacement device to the system at
the same time as the original device is working, and then issue the
replace command. I think this technique is safe, as the original device
is still available during the replacement procedure and could be used to
provide redundancy to the rest of the pool until the new device finishes
resilvering. (Does anyone know if this is really the case...i.e. if
redundancy is preserved during the replacement operation when both
original and new devices are connected simultaneously and both are
functioning correctly? One way to verify this is might be to run zfs
replace on a non-redundant pool while both devices are connected.)
The second way is to (physically) disconnect the original device and
connect the new device in its place. The pool will be degraded because
a member device is missing...if you have RAIDZ1, you have no redundancy
remaining, if you have RAIDZ2, you still have 1 level of redundancy
intact. The zfs replace command should be able to rebuild the missing
data onto the replacement new device.
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