Using LVM snapshots

One of the reasons we created a second layer of LVM on top of DRBD was to provide filesystem snapshot capabilities. When we create a snapshot, all files on a particular volume will appear static on that snapshot until one of the following two things happens:

• We destroy the snapshot
• The amount of changes on the source volume is larger than the space we reserved for the snapshot

This is the primary reason we left 5 percent space unused within our PostgreSQL volume group. If we create a snapshot, up to 5 percent of the database can change before we have to remove it. For larger storage devices, this should give us a lot of time to perform emergency restores, create byte-stable backups, or any other operation that requires consistent data.

In this recipe, we'll learn how to properly allocate, use, and remove an LVM snapshot.

Getting ready

For this recipe, we want a formatted and active XFS filesystem. Please follow the recipe in Formatting an XFS filesystem before continuing...