Lesson 3.70: LVM snapshots before risky changes

Some changes rewrite data in place: a database migration, a vendor's upgrade script, a bulk edit. If they go wrong, restoring from last night's backup loses today's work and takes time. An LVM snapshot freezes a logical volume's state in seconds, using only the space of the blocks that change afterwards; if the change fails you merge the snapshot back and the volume returns to that moment. In the lab you snapshot /srv/data before a vendor's catalog migration, watch the migration break the catalog and fill the snapshot, and roll back.

16 min

You will learn to

Explain copy-on-write snapshots
Create a snapshot with lvcreate -s
Read snapshot usage (Data%) in lvs
Roll back with lvconvert --merge
Know why a snapshot isn't a backup

Key terms

Snapshot: A logical volume that preserves another volume's contents at one moment.
Copy-on-write: Before a block of the origin is overwritten, its old content is copied into the snapshot.
Origin: The volume a snapshot was taken of.
Data%: How full a snapshot's space is; at 100% the snapshot becomes invalid.
Merge: Writing a snapshot back onto its origin: a rollback.

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