Caché High Availability Guide - InterSystems Documentation

unwieldy. The approach is to freeze writes to all database files for the duration required tocreate a snapshot, then create a snapshot of the disk using the technology of choice. Whenthe snapshot has been created, the system is thawed to allow writes to continue to the databasewhile the snapshot image is copied to the backup mediaWhile the ability to do this has always existed in Caché, this release provides theBackup.General class with class methods to simplify and enhance this technique. Using thenew class methods on a nonclustered instance of Caché, only physical writes to the databaseare paused during the creation of the snapshot, while user processes are allowed to continueperforming updates in memory. This allows for a zero-downtime external backup on nonclusteredsystems. You should be careful to ensure that this mechanism is being used with a disktechnology that can create the snapshot within several minutes. If writes are paused for anextended period of time, user processes could hang due to a shortage of free global buffers.On a clustered configuration of Caché, this method pauses user processes for the duration ofthe freeze.In addition to pausing writes as described above, the freeze method also handles switchingjournal files, and writing a backup marker to the journal. The class methods that perform thedatabase freeze and thaw operations are Backup.General.ExternalFreeze() andBackup.General.ExternalThaw() respectively. See the Backup.General class documentationin the Caché Class Reference for details on the use of these methods.Caché maintains a bitmap list of database blocks modified since the last backup; you can usethis list to keep track of database updates during backup stages. The DBSIZE utility, whichinspects these bitmaps, helps you calculate the size of a backup. Caché BACKUP uses amultipass scan of these lists to back up only the blocks modified since the last pass. Subsequentpasses usually operate on a reduced list of modified blocks; generally three passes are sufficientto complete a backup. During the final pass, Caché suspends all processes to prevent updatesand allow the backup to completely copy the last short list of modified blocks. This methodis used in most of the backup strategies that follow.The following sections discuss the types of external backups and the advantages and disadvantagesof each:• Concurrent External Backup• Non-concurrent External Backup• Cold BackupBackup MethodsCaché High Availability Guide 19