Zvirt = Libvirt ZFS snapshots

Purpose

Zvirt takes snapshots of Libvirt domains using ZFS. It supports both crash-consistent and live snapshots.

At the end, all components of a domain - Domain definition, TPM, NVRAM, VirtioFS, disks (either files on a ZFS dataset or raw zvols) - are captured as a set of consistent ZFS snapshots.

Features

• Take snapshots of Libvirt domains using ZFS.
• Support both crash-consistent and live snapshots.
• Support batch mode (pause all domains, take snapshots, then resume all domains)

Snapshot retention

snapshot-libvirt-domains runs zfs-autobackup with --no-thinning: it only creates snapshots and never deletes any. Retention is left to a separate zfs-autobackup run in snapshot-only mode:

$ zfs-autobackup --no-snapshot --keep-source 1w1d,1m1w,1y1m \
                 --snapshot-format "$(snapshot-libvirt-domains --print-snapshot-format)" libvirt

That run must be given the very same snapshot name format zvirt snapshotted with: zfs-autobackup only thins snapshots whose name parses against the format it was given. On a mismatch it selects the right datasets, matches none of their snapshots, deletes nothing and still exits 0 with All operations completed successfully — so never hardcode the format, always query it.

Two ways to query it:

$ snapshot-libvirt-domains --print-snapshot-format
libvirt-%Y-%m-%d-%H:%M:%S

$ . /usr/share/zvirt/snapshot-format && echo "$ZVIRT_SNAPSHOT_FORMAT"
libvirt-%Y-%m-%d-%H:%M:%S

/usr/share/zvirt/snapshot-format is the single source of truth: snapshot-libvirt-domains sources it too, so the format cannot drift between the tool that writes the snapshots and the tool that prunes them.

The format deliberately carries no {} placeholder (zfs-autobackup's default is {}-%Y%m%d%H%M%S, where {} expands to the backup name). All domains therefore share a single snapshot name pattern, which is what allows one prune run over the shared autobackup:libvirt property to cover every domain at once, instead of one invocation per domain.

Pruning from a systemd unit

%Y, %m, %d, %H, %M and %S are systemd specifiers: written literally in an ExecStart= they are silently expanded into paths and hostnames, and systemd-analyze verify does not catch it. Either double them (%%Y, %%m, ...) or — better — let the unit call a wrapper that queries the format:

ExecStart=/bin/bash -c '. /usr/share/zvirt/snapshot-format; exec /usr/bin/zfs-autobackup --no-snapshot --keep-source 1w1d,1m1w,1y1m --snapshot-format "$ZVIRT_SNAPSHOT_FORMAT" libvirt'

License

MIT License

Author

Nicolas Massé