HP StorageWorks XP Remote Web Console Software - Manual (Page 12)

12 Overview of copy solutions
Figure 1 illustrates the mechanism Snapshot XP uses to copy data in the array.
Figure 1 Snapshot XP mechanism
In Figure 1 , Data A is stored in the Snapshot XP pair’s P-VOL. When the host requests to update Data A to
Data B, Snapshot XP copies Data A to the pool volume (pool-VOL) in the pool before updating the P-VOL.
After the copy operation for Data A is complete, Snapshot XP writes Data B to the area where Data A was
stored in the P-VOL, and the P-VOL is updated. From the S-VOL, you can access Data B in the P-VOL and
Data A in the pool-VOL.
Snapshot XP and Business Copy XP are similar because both create pairs. However, Snapshot XP pair
S-VOLs do not consume array capacity because the S-VOLs are actually virtual volumes. Therefore,
Snapshot XP reduces the capacity the array uses as a whole.
The copy process for Business Copy XP pairs takes longer because all P-VOL data is copied to the S-VOL.
On the other hand, the copy process for Snapshot XP pairs takes less time compared to the copy process
for Business Copy XP pairs because, as you can see in Figure 1 , only updated P-VOL data is copied to the
pool-VOL. The same data in the P-VOL can be referenced by accessing the Snapshot XP pair’s S-VOL, but
the actual data does not exist in the S-VOL. The P-VOL data can be referenced using the S-VOL.
As described previously, Snapshot XP requires less capacity in the array and less copy processing time
compared to Business Copy XP. HP recommends using Snapshot XP to copy and manage data in the short
term with reduced costs. However, Snapshot XP only copies some of the P-VOL data. Therefore, data
stored in the S-VOL is not guaranteed in the following:
• When a physical failure occurs in a P-VOL
• When a pool is blocked
On the other hand, Business Copy XP copies all P-VOL data to the S-VOL. Therefore, even if a physical
failure occurs in a P-VOL, you can recover the P-VOL data using the S-VOL data. Business Copy XP provides
higher integrity than Snapshot XP. HP recommends using Business Copy XP when data integrity is more
important than copy process speed or array capacity. For a functional overview of Business Copy XP, see
” Business Copy XP ” on page 13.