Utilizing snapshot is an effective method for restore of previous versions of data and deleted files. Snapshot utilize hardlinks and only changed and deleted files are saved as separate files in a snapshot. Files which are not changed are hardlinks to the original file.

If a file.txt is saved in snapshot number one and never changed or deleted, the file file.txt in the latest snapshot is just a hardlink to the original file. If the file.txt is deleted from the first snapshot, the filesystem takes care of updating and where to save the original file as part of the delete operation.

Even if all snapshots are tagged for delete, the first and last snapshot are not deleted. The first and last snapshot are removed from the delete list as part of preparation (internal) of delete.

Snapshot is not possible in a rsync daemon setup.

What is a snapshot?

A snapshot is a saved state or backup of data at a specific point of time. Every snapshot is in sync with local catalog at the time of creating the snapshot. Previous versions of files can be restored from a snapshot. The snapshot is by utilizing the --link-dest parameter to rsync. The rsync parameter for next snapshot to save is:

--link-dest=~/snapshots/n-1 /Volumes/user/data/ user@remote.server:~/snapshots/n

where

  • n is the number of next snapshot to be saved
  • n-1 is the latest saved snapshot
  • /Volumes/user/data/ is the source catalog
  • ~/snapshots/ is the remote catalog where snapshots are saved, the remote catalog is set by the user

If remote catalog is a local volume full path must be added. The source catalog is never touched, only read by rsync.

RsyncOSX creates the snapshots within the remote catalog. The ~ is expanded to the user home catalog on remote server. Utilizing snapshot on local attached disks require full path for remote catalog.

~/snapshots/1

  • snapshot one
  • a full sync when snapshot is created

…

~/snapshots/n

  • snapshot n
  • n is the latest snapshot to be saved

Create a snapshot

To create a snapshot task select snapshot as type in add tasks. Do not copy and paste command for execution within a terminal window. RsyncOSX saves the number n to the configuration. The number n is the next snapshot number.

Snapshot administration

It is important to administrate snapshots. By administrate means deleting not relevant snapshots. If snapshots are never deleted the number of snapshots might become difficult to use. A snapshot is most likely used to restore old and deleted files. This is why a plan to administrate snapshots is important. RsyncUI can assist you in this.

Deleting snapshots is a destructive operation and should be performed with care. It is important to have a plan about which snapshots to keep and which to delete. RsyncUI utilizes a simple plan for delete and keep snapshots.

The plan for keep and delete

Selecting the Tag button evaluates all snapshots based on the date withing the log record. Based and the selected plan and date, snapshots are either tagged with keep or delete. Snapshots which are tagged with delete are also preselected for delete. To actually delete the marked snapshots require to select the Delete button.

Even if all snapshots are tagged for delete, the first and last snapshot are not deleted. The first and last snapshot are removed from the delete list as part of preparation (internal) of delete.

The plan is based upon three parts where the parameter plan has an effect on previous months (and years):

  • the current week
    • keep all the snapshots within the current week
    • value of plan has no effect on the current week
  • the current month minus the current week
    • keep all snapshots for the selected Day of week, e.g all snapshots every Sunday this month
    • value of plan has no effect on the current month
  • previous months (and years)
    • keep the snapshot in the last week of month for selected Day of week, e.g the last Sunday in the month
    • if plan == Every, keep for the selected Day of week, e.g all snapshots every Sunday, every week in previous period
    • if plan == Last, keep for the selected Day of week, e.g all snapshots every last Sunday every month in previous period

Tagging and delete snapshots

It is advised to cleanup the number of snapshots. Select a plan, tagg the snapshots and delete the snapshots which are marked for delete.