This section describes the replication function EC (Equivalent Copy) for ETERNUS DX/AF which offers the clone functionality.
Overview
EC creates a copy of the original unit at a particular time ("Point-in-Time copy"). The copy, which is called a clone unit, is available immediately after it has been activated. However, it can only be activated after the copy operation between the original and the copy has been completed.
Together the original unit and clone unit form the clone pair which is managed by EC.
For simplicity’s sake, the term unit is generally used in this manual instead of original unit. A clone pair therefore consists of a unit and a clone unit.
EC does not require any special preconfigured devices. Any units of the same size of the storage system can be used as clone units. After activation, the unit and clone unit are accessible, which enables both to be processed separately, for example by different applications. After separate processing has been completed, the copy containing the original data can be updated to restore their identical status.
Write accesses to the unit in mirrored status also take place simultaneously to the clone unit.
For the duration of initial synchronization that can have repercussions on the performance of the application which accesses data on the original unit. The clone unit (also referred to as local mirror) can only be activated when synchronization has been completed.
When replication is interrupted or after the pair has been canceled, both the unit and the clone unit can be accessed by the server.
When replication is resumed, only the modified data is copied from the unit to the clone unit.
Direct reconstruction (copying back) from a clone unit to the unit (restore) is possible.
If the copy is discarded, the clone session is terminated and the clone unit can be used freely again.
SHC-OSD offers the option of consistent generation of a pubset’s clone units during ongoing operation. Stopping all inputs/outputs on all the units concerned while splitting is in progress ensures data consistency. Among other things, this can be used to create a consistent standby pubset from the home pubset.
Figure 11: Equivalent Copy
Application
The clone units which are provided by EC supply copies of the productive application data. Data backups, batch evaluations and program tests can consequently be executed on the copies of the “live data.” The original data is available in parallel to the main application, and as a result the latter can remain productive longer.
The copies generated with EC can be used for data backup (e.g. with HSMS or FDDRL). Replicas of databases can be generated with clone units.
Migration
The EC function is ideally suited to migrating data. To do this, a clone session is set up between the unit with the data to be migrated and the target unit (= clone unit), if required between multiple original and multiple target units.
After the copy procedure has been completed and the clone unit has been activated, the data is available on the new clone unit. The clone session is now terminated and the clone unit can be used with the migrated data.
Device configuration
Special configuration of the devices for EC is not required. The devices can be mapped to BS2000 mnemonic names when at least one device is connected to the server.
Clone units
Freely available devices can be used as clone units provided they match the devices to be copied in terms of capacity and device type (D3435). The RAID level can differ.
Like any other disk, the units used as clone units must be generated in BS2000.
At present up to 32 clone units can be configured for a unit. This maximum number includes the total value for all replication functions, i.e. QuickOPC, EC, SnapOPC+ and REC. The SnapOPC+ sessions count as one replication. If the RESTORE function should be used, a replication has to be reserved for it.
Storage Cluster
The “Continuous Copy” function can not be used in a Storage Cluster if the local replication is executed with EC, see chapter "Storage Cluster Option".