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.
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.
What is a snapshots?
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 snapshots. 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/ email@example.com:~/snapshots/n
nis the number of next snapshot to be saved
n-1is 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/2- snapshot two
- the next snapshots saves the changed files and makes hard links for files not changed
~/snapshots/n-1- snapshot n
- n-1 is the latest snapshot saved to disk
~/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 tab.
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. The number n is used when computing the parameter for rsync and is picked up from the configuration.
It is important to administrate snapshots. By administrate means deleting 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 are important. RsyncOSX can assist you in this.
To administrate snapshots select the snapshot tab. 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. RsyncOSX utilizes a simple plan for delete and keep snapshots. The plan is based upon three parts:
- the current week
- keep all the snapshots within 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
- 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
Snapshots which does not follow plan are automatically marked for deleting. To actually delete the marked snapshots require to select the Delete button.