HP XP7 Storage - Manual (Page 11)

Figure 5 On-demand copy (case 3: Write request to an area in the T-VOL to which the data has
not been copied)
1. From the host, there is a write request to an S-VOL from where the data has not been copied,
or there is a Read or Write request to the space in the T-VOL to where the data has not been
copied.
2. When an operation as described is made to the space in the S-VOL or T-VOL where the data
has not been copied, Compatible FlashCopy performs on-demand copying before the Read
or Write operation. By on-demand copying, the data is copied from the S-VOL to the T-VOL.
3. When on-demand copying is completed, the Read or Write operation is performed.
On-demand copy may not always be executed when there is a read request from the host. In that
case, the host directly reads the S-VOL data instead of the T-VOL data.
WARNING! When the NOCOPY mode is selected, even if all the data in the S-VOL is copied
to the T-VOL by on-demand copy processing, Compatible FlashCopy does not automatically
withdraw the relationship. Therefore, when you selected NOCOPY mode, you must withdraw the
relationship explicitly by using the FCWITHDR command. If you select the NOCOPY2COPY mode
for a relationship made using NOCOPY mode, the background copying process is performed.
When all data is copied, the relationship is released automatically.
Copying by volume
If you do not specify an extent when you establish a relationship, the entire volume is copied. This
is called volume copying. To perform volume copying, the capacity of the copy target volume must
be equal to or larger than the capacity of the copy source volume.
Copying by dataset (Compatible FlashCopy only)
If you specify the extent when you establish a relationship, only a part of the volume will be copied.
This is called dataset copying. Dataset copying establishes the relationship only using the specified
extent or extents. You can specify up to 16 target extents for one source extent. To perform dataset
copying, the number of tracks in the copy source extent and the copy target extent must be the
same.
The following list shows the available and unavailable combinations of extents to establish
relationships in the case of dataset copying.
Establishing a Compatible FlashCopy relationship 11