ZFS Snapshots¶
Overview¶
Every ZFS-based PetaLibrary allocation has snapshots enabled by default. ZFS snapshots are read-only representations of a ZFS filesystem at the time the snapshot is taken. Many allocations still reside in BeeGFS where snapshots are not available; we expect to complete migrations from BeeGFS to ZFS by the end of 2021.
How to access snapshots¶
Snapshots are accessible in the root of the filesystem. If, for example,
your allocation is located in /pl/active/rcops, your snapshots are
accessible in /pl/active/rcops/.zfs/snapshot. Note that the .zfs directory
is not visible to any utilities that list directory contents, so an ‘ls’
of the .zfs directory will fail, although you can ‘cd’ to it.
Snapshots times are in UTC, which is not affected by daylight savings time. To convert from UTC to MDT, subtract six hours, and to convert from UTC to MST, subtract seven hours.
How snapshots affect free space in your allocation¶
PetaLibrary allocation sizes are set with quotas, and snapshot use does count against your quota. Removing a file from your filesystem will only return free space to your filesystem if no snapshots reference the file. Filesystem free space does not increase until a file on a filesystem and all snapshots referencing said file are removed. Because snapshots can cause confusion about how space is utilized within an allocation, the default snapshot schedule discards snapshots that are more than one week old.
Default snapshot schedule¶
The default snapshot schedule is as follows:
- take snapshots every 15 minutes, save most recent 9 snapshots (two hours worth)
- take snapshots every hour, save most recent 25 snapshots (one day worth)
- take snapshots every day, save more recent 8 snapshots (one week worth)
If you would like to set a custom snapshot schedule for your allocation, please contact rc-help@colorado.edu. Note that the longer you retain snapshots, the longer it will take to free up space by deleting files from your allocation.