Setting Reservations on ZFS File Systems

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.