68Understanding Veritas Volume Manager

FastResync

Availability (HA) environment requires the full resynchronization of a mirror when it is reattached to its parent volume.

How non-persistent FastResync works with snapshots

The snapshot feature of VxVM takes advantage of FastResync change tracking to record updates to the original volume after a snapshot plex is created. After a snapshot is taken, the snapback option is used to reattach the snapshot plex.

Provided that FastResync is enabled on a volume before the snapshot is taken, and that it is not disabled at any time before the snapshot is reattached, the changes that FastResync records are used to resynchronize the volume during the snapback. This considerably reduces the time needed to resynchronize the volume.

Non-Persistent FastResync uses a map in memory to implement change tracking. Each bit in the map represents a contiguous number of blocks in a volume’s address space. The default size of the map is 4 blocks. The kernel tunable vol_fmr_logsz can be used to limit the maximum size in blocks of the map as described on “Tunable parameters” on page 475.

Persistent FastResync

Unlike non-persistent FastResync, persistent FastResync keeps the FastResync maps on disk so that they can survive system reboots, system crashes and cluster crashes. Persistent FastResync can also track the association between volumes and their snapshot volumes after they are moved into different disk groups. When the disk groups are rejoined, this allows the snapshot plexes to be quickly resynchronized. This ability is not supported by non-persistent FastResync. See “Reorganizing the contents of disk groups” on page 195 for details.

If persistent FastResync is enabled on a volume or on a snapshot volume, a data change object (DCO) and a DCO volume are associated with the volume.

DCO volume versioning

The internal layout of the DCO volume changed in VxVM 4.0 to support new features such as full-sized and space-optimized instant snapshots. Because the DCO volume layout is versioned, VxVM software continues to support the version 0 layout for legacy volumes. However, you must configure a volume to have a version 20 DCO volume if you want to take instant snapshots of the volume. Future releases of Veritas Volume Manager may introduce new versions of the DCO volume layout.

See “Determining the DCO version number” on page 277 for a description of how to find out the version number of a DCO that is associated with a volume.