Third-Party Storage Snapshots

A third-party storage snapshot is a set of pointers to storage blocks (not Oracle blocks) that existed when the snapshot was created. The virtual copies reside on the same storage array as the original data. Figure 1-4 depicts a copy-on-write snapshot, which is a type of third-party snapshot. After a snapshot is taken, when the first change to a storage block occurs, the array copies the before-image block to a new location on disk (C) and writes the new block (C') to the original location.

Figure 1-4 Third-Party Copy-on-Write Snapshot

Description of "Figure 1-4 Third-Party Copy-on-Write Snapshot"

This technique has the following advantages:

An initial copy of the database is not necessary because snapshots are not stored as physical copies of blocks. Thus, less storage is consumed than in RMAN strategies.
Snapshots can be extremely fast. You put the database in backup mode (unless storage does not meet the requirements for snapshot storage optimization), and then take the snapshot. The snapshot needs to store physical blocks only when the blocks change, so a backup of an unchanged file is a metadata-only operation.
Snapshots use storage efficiently. A backup of a file with a single changed block requires only one additional version of the block to be stored—either the old version or new version of the block, depending on the snapshot technique.

Some disadvantages are as follows:

Snapshots have no knowledge of an Oracle Database block structure, and thus cannot validate Oracle blocks.
Because snapshots reside on the same storage array as the source database, they are vulnerable to storage failures and data corruptions. If the array is inaccessible, or if the storage contains data block corruptions, then the snapshots cannot be used for recovery.
Restoring a snapshot in place voids all snapshots that were taken after it unless the snapshot is fully restored to an alternate location.