To create a snapshot, use the zfs snapshot subcommand. Since snapshot operations occur on a file system, rather than on the disks that comprise a pool, the zfs command is used to manage snapshots. Subsequent articles will demonstrate snapshot clones, rollback, and other advanced features of snapshots. In this article we’ll start with the basics: creating, using, and deleting file system snapshots. Since snapshots record the differences between the time the snapshot was taken and the current state of the file system, the size of a snapshot increases over time, reflecting the size of the changes. Because ZFS is a COW (copy-on-write) file system, the initial size of a snapshot is 0 bytes.
The ability to access files from a certain point in time sounds great, but doesn’t that take a lot of storage space? Not necessarily. While snapshots do not replace backups, they provide a quick and convenient method for accessing files from a specific point in time. Restoring data from a snapshot is quicker than restoring from a backup or waiting for a system administrator to restore a backup for you. Need to test some configuration changes but don’t want to permanently muck up the system? Or perhaps you’re preparing for an operating system upgrade and want a quick way to revert to the pre-upgrade version should things go terribly wrong. Perhaps you’re considering updating an application but fear the pain of clobbered libraries. Need a copy of that file you deleted last Tuesday? No problem. Imagine taking a snapshot of your home directory at the beginning of the work day, and perhaps another one after lunch.
This is a powerful feature as there are many scenarios where it is convenient to access files from a certain point in time. What exactly is a snapshot? zfs(8) defines it as a “ Read-only version of a file system … at a given point in time”. Since snapshots are atomic, they are not affected by other processes and you don’t have to stop any running applications before taking a snapshot. Snapshots complement a backup strategy, as they are instantaneous and don’t require a backup window. OpenZFS stands out in its snapshot design, providing powerful and easy-to-use tools for managing snapshots.
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