Maximizing Efficiency Through Data Replication Manager Data Movement Application Notes

Data Backup
Application Notes - Maximizing Efficiency Through Data Replication Manager Data Movement 5
Figure 1: Backup concentration model
Disk-to-Disk Backup
A clone is a point-in-time exact copy of a Just a Bunch of Disks (JBOD), mirrorset, stripeset, or striped
mirrorset that can be physically moved to an archival facility (see Figure 2 for the steps required when
using the CLONE utility). RAIDsets (3/5) cannot be cloned. One application that simplifies the backup
process is the Compaq Enterprise Volume Manager (EVM). This software has the ability to create
clones of target units with minimal impact to the application servers. EVM Version 2.0 has the ability
to support multiple storage subsystems. This means that EVM in a DRM configuration requires one
server for each site.
With normal two-member mirrorsets or striped mirrorsets, EVM creates a third copy of the data. This
copy is separated from the mirrorset and can be made into a unit and mounted to a system, or can be
removed to a fireproof vault for safe keeping. Any future changes to the mirrorset would not be written
to the clone. While this method is faster than tape backup, the process of copying many gigabytes of
data to the clone still requires a considerable amount of time. This time is transparent to the application
server because its operation is only halted when the clone is split off, which takes only a few seconds to
complete.
The lengthy cloning process can be considerably shortened with the use of the three-member mirrorset
or striped mirrorset. EVM can detect that there are three members to the set and instead of creating a
fourth copy and splitting it from the set, it simply removes the third member and makes it the clone.
This can be accomplished in seconds rather than in hours. Compaq highly recommends that a
three-member mirrorset or striped mirrorset be used as the target site’s remote copy set whenever data
vaulting (storage) of disks occurs. (Refer to Figure 3.) Data across each stripe is duplicated three times
during the normal operation using a single write command and the clone can be made instantaneously.
There is little additional overhead for the third member.
EVM provides the ability to quiese the data on the clone and provide crash consistent data. Attempting
to manually remove the third member of a mirrorset would require stopping the database for a period
of time to remove the mirror member and to add back the replacements. A manual removal would not
ensure crash consistent data.
Site A
Site C
Site B
CXO7696A
Backup Site
Note: Data transfer to a backup site would occur sequentially, and not
simultaneously as depicted.