VERITAS Storage Foundation 4.1 for Oracle RAC HP Serviceguard Storage Management Suite Extracts, December 2005
Using Storage Checkpoints and Storage Rollback for Backup and Restore
A direct application of the Storage Checkpoint facility is Storage Rollback. Because each
Storage Checkpoint is a consistent, point-in-time image of a file system, Storage Rollback
is the restore facility for these on-disk backups. Storage rollback rolls back blocks
contained in a Storage Checkpoint into the primary file system for faster database
recovery. For more information on Storage Checkpoints and Storage Rollback, see the
VERITAS File System Administrator’s Guide.
Understanding Storage Checkpoints and Storage Rollback
A Storage Checkpoint is a disk and I/O efficient snapshot technology for creating a
“clone” of a currently mounted file system (the primary file system). Like a snapshot file
system, a Storage Checkpoint appears as an exact image of the snapped file system at the
time the Storage Checkpoint is made. However, unlike a snapshot file system that uses
separate disk space, all Storage Checkpoints share the same free space pool where the
primary file system resides. A Storage Checkpoint can be mounted as read-only or
read-write, allowing access to the files as if it were a regular file system.
Initially, a Storage Checkpoint contains no data—it contains only the inode list and the
block map of the primary fileset. This block map points to the actual data on the primary
file system. Because only the inode list and block map are needed and no data is copied,
creating a Storage Checkpoint takes only a few seconds and very little space.
A Storage Checkpoint initially satisfies read requests by finding the data on the primary
file system, using its block map copy, and returning the data to the requesting process.
When a write operation changes a data block n in the primary file system, the old data is
first copied to the Storage Checkpoint, and then the primary file system is updated with
the new data. The Storage Checkpoint maintains the exact view of the primary file system
at the time the Storage Checkpoint had been taken. Subsequent writes to block n on the
primary file system do not result in additional copies to the Storage Checkpoint because
the old data only needs to be saved once. As data blocks are changed on the primary file
system, the Storage Checkpoint gradually fills with the original data copied from the
primary file system. Less of the block map in the Storage Checkpoint points back to blocks
on the primary file system.
Storage rollback restores a database, a tablespace, or datafiles in the primary file systems
to the point-in-time image created during a Storage Checkpoint. Storage rollback is
accomplished by copying the “before” images from the appropriate Storage Checkpoint
back to the primary file system. As with Storage Checkpoints, Storage Rollback restores at
the block level, rather than at the file level.
If you mount a Storage Checkpoint as read-write, the CLI utility mounts the Storage
Checkpoint as read-only. It then creates a shadow Storage Checkpoint of the mounted
read-only checkpoint and mounts the shadow Storage Checkpoint instead. This ensures
that any Storage Checkpoint data that has been modified incorrectly cannot be a source of
any database corruption.
130 VERITAS Storage Foundation for Oracle RAC Installation and Configuration Guide