HP StorageWorks Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide (AA-RPHZF-TE, March 2004)

Replicating Storage Units
219Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide
ADD UNIT D99 C_ST1
D99 HAS BEEN CREATED. IT IS A CLONE OF D98.
CLONE - NORMAL TERMINATION
Snapshot
With snapshot, the contents of a source unit are frozen in time and presented to the host as a
second unit, the snapshot. The snapshot unit (Figure 37) preserves the original data (from the
time of the snapshot) while allowing writes to the source unit to continue. A temporary volume
(the snapshot unit) is created and used to store the original data that has been overwritten on
the source unit since the time of the snapshot.
Using a cache bitmap, reads are directed to the source unit or the snapshot unit.
If no data has been written to the source unit since the time of the snapshot, data is then read
from the source unit.
If data has been written to the source unit, then data is read from the snapshot unit.
Figure 37: Snapshot unit
Server
Source Unit
Write
new data
Controller cache
bitmap check
Snapshot unitSource unit
Write
old data
Before Snapshot created
and after Snapshot deleted
Write operations to a Source Unit
that has a Snapshot
Read operations to Snapshot Unit
Server
0 to 1
Read Read
Read
Controller cache
bitmap check
Snapshot unitSource unit
Server
01
0 = Data on Source Unit at time of Snapshot is unchanged
1 = Data on Source Unit at time of Snapshot transferred to
Snapshot Unit
CXO7300A