Q

Copy-on-write snapshots vs. differencing disk snapshots

Independent backup expert Brien Posey compares copy-on-write snapshots and differencing disk snapshots in this Expert Response.

Copy-on-write snapshots require more storage capacity than differencing disk snapshots, but how much more? Is there

a way to calculate the difference?

The reason copy-on-write snapshots consume so much more storage space than differencing-disk snapshots is that copy-on-write snapshots retain copies of storage blocks, even after the data has been deleted or modified. This allows the snapshot software to revert a file or, in some cases, even an application to an earlier point in time without having to roll back the entire volume.

Unfortunately, there is no universal guideline for how much storage copy-on-write snapshots will require. The space required varies, depending on how rapidly your data changes and on your snapshot retention range.

You can get a feel for the rate of change by performing traditional differential backups and monitoring the size of those backups, but you must keep in mind that snapshots will typically consume more space than a differential backup will reflect. The reason for this is that a differential backup backs up your data as it exists at a point in time. The snapshot process is ongoing and in a 24-hour period, a snapshot will likely copy more data than a traditional differential backup would.

This was first published in November 2012

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