Backup in a snap: A guide to snapshot technologies


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Different types of snapshots and how they work

There are six general types of snapshot technologies:

  1. Copy-on-write
  2. Redirect-on-write
  3. Clone or split-mirror
  4. Copy-on-write with background copy
  5. Incremental
  6. Continuous data protection

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Copy-on-write (COW) snapshot

COW requires storage capacity to be provisioned for snapshots, and then a snapshot of a volume has to be initiated using the reserved capacity. The COW snapshot stores only the metadata about where the original data is located, but doesn't copy the actual data at the initial creation. This makes snapshot creation virtually instantaneous, with little impact on the system taking the snapshot.

The snapshot then tracks the original volume paying attention to changed blocks as writes are performed. As the blocks change, the original data is copied into the reserved storage capacity set aside for the snapshot prior to the original data being overwritten. The original data blocks snapped are copied just once at the first write request. This process ensures snapshot data is consistent with the exact time the snapshot was taken, and it's why the process is called "copy on write."

Read requests to unchanged data are directed to the original volume. Read requests to changed data are directed to the copied blocks in the snapshot. Each snapshot contains metadata describing the data blocks that have changed since the snapshot was first created.

The major advantage of copy-on-write is that it's incredibly space efficient because the reserved snapshot storage only has to be large enough to capture the data that's changed. But the well-known downside to copy-on-write snapshot is that it will reduce performance on the original volume. That's because write requests to the original volume must wait to complete until the original data is "copied out" to the snapshot. One key aspect of copy-on-write is that each snapshot requires a valid original copy of the data.

This was first published in October 2009

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