is it possible to convert dataset to sparse volume (thin provisioning)?
I don't think the accepted answer is correct in fact, but I don't have enough rep to comment.
Assuming when the OP says "datasets" they mean "ZFS volume" (or ZVOL).
The only difference between a thin-provisioned (a.k.a sparse) ZVOL and a regular one is whether the full size is reserved via the refreservation
property e.g.
# zfs create -V 10G tank/test_full
# zfs create -s -V 10G tank/test_sparse
# zfs list -o name,used,usedbydataset,usedbyrefreservation,logicalused,logicalreferenced,refreservation tank/test_sparse tank/test_full
NAME USED USEDDS USEDREFRESERV LUSED LREFER REFRESERV
tank/test_full 10.3G 136K 10.3G 30K 30K 10.3G
tank/test_sparse 136K 136K 0 30K 30K none
So you can convert an existing ZVOL to sparse by changing refreservation
for an existing ZVOL e.g.
# zfs set refreservation=none tank/test_full
# zfs list -o name,used,usedbydataset,usedbyrefreservation,logicalused,logicalreferenced,refreservation tank/test_sparse tank/test_full
NAME USED USEDDS USEDREFRESERV LUSED LREFER REFRESERV
tank/test_full 136K 136K 0 30K 30K none
tank/test_sparse 136K 136K 0 30K 30K none
As far as I know it's not possible to convert an existing zvol to sparse; they must be created sparse to begin with.
However, you can create a new sparse zvol, and zfs send
from the old one and zfs recv
to the new one, and then swap their names.