When you use the Advanced Open File Option with the option, Automatically select open file technology, Backup Exec chooses either the Symantec Volume Snapshot Provider or the Microsoft Volume Shadow Copy Service as the technology to use when open files are encountered. Both of these providers use "Copy-on-Write" technology to create snapshots. The snapshot itself consists of a virtual volume and a cache file. The cache file keeps track of the changes to the volume being snapped after the snapshot is taken. Using this technology, Backup Exec can back up data at a point-in-time, while also ensuring data consistency.
The size of the cache file grows based on the amount of time the snapshot is active and by the rate of data change occurring on the volume during the time the snapshot is active. By default, Backup Exec sets a predetermined maximum cache file size, which in cases of heavy disk write activity during a backup job, can be exceeded. In such cases, the backup job will fail. If this occurs, you can increase the maximum size of the Advanced Open File Option cache file using the Advanced Open File Option wizard, or by running backup jobs during periods of low computer usage. In extreme cases, you may have to allocate a maximum cache file size equal to the used space on the volume being snapped.