A ZFS reservation is an allocation of space from the pool that is guaranteed to be available to a dataset. As such, you cannot reserve space for a dataset if that space is not currently available in the pool. The total amount of all outstanding unconsumed reservations cannot exceed the amount of unused space in the pool. ZFS reservations can be set and displayed by using the zfs set and zfs get commands. For example:
# zfs set reservation=5G tank/home/moore # zfs get reservation tank/home/moore NAME PROPERTY VALUE SOURCE tank/home/moore reservation 5G local |
ZFS reservations can affect the output of the zfs list command. For example:
# zfs list NAME USED AVAIL REFER MOUNTPOINT tank/home 5.00G 33.5G 8.50K /export/home tank/home/moore 15.0K 33.5G 8.50K /export/home/moore |
Note that tank/home is using 5 Gbytes of space, although the total amount of space referred to by tank/home and its descendents is much less than 5 Gbytes. The used space reflects the space reserved for tank/home/moore. Reservations are considered in the used space of the parent dataset and do count against its quota, reservation, or both.
# zfs set quota=5G pool/filesystem # zfs set reservation=10G pool/filesystem/user1 cannot set reservation for 'pool/filesystem/user1': size is greater than available space |
A dataset can use more space than its reservation, as long as space is available in the pool that is unreserved and the dataset's current usage is below its quota. A dataset cannot consume space that has been reserved for another dataset.
Reservations are not cumulative. That is, a second invocation of zfs set to set a reservation does not add its reservation to the existing reservation. Rather, the second reservation replaces the first reservation.
# zfs set reservation=10G tank/home/moore # zfs set reservation=5G tank/home/moore # zfs get reservation tank/home/moore NAME PROPERTY VALUE SOURCE tank/home/moore reservation 5.00G local |
You can set a refreservation to guarantee space for a dataset that does not include space consumed by snapshots and clones. The refreservation reservation is accounted for in the parent datasets' space used, and counts against the parent datasets' quotas and reservations. For example:
# zfs set refreservation=10g profs/prof1 # zfs list NAME USED AVAIL REFER MOUNTPOINT profs 10.0G 23.2G 19K /profs profs/prof1 10G 33.2G 18K /profs/prof1 |
You can also set a reservation on the same dataset to guarantee dataset space and snapshot space. For example:
# zfs set reservation=20g profs/prof1 # zfs list NAME USED AVAIL REFER MOUNTPOINT profs 20.0G 13.2G 19K /profs profs/prof1 10G 33.2G 18K /profs/prof1 |
Regular reservations are accounted for in the parent's used space.
In the above example, the smaller of the two quotas (10 Gbytes versus 20 Gbytes) is displayed in the zfs list output. To see the value of both quotas, use the zfs get command. For example:
# zfs get reservation,refreserv profs/prof1 NAME PROPERTY VALUE SOURCE profs/prof1 reservation 20G local profs/prof1 refreservation 10G local |
If refreservation is set, a snapshot is only allowed if enough free pool space exists outside of this reservation to accommodate the current number of referenced bytes in the dataset.