HP P9000 for Compatible FlashCopy Mirroring User Guide (AV400-96388, October 2011)

If there is a request for a write operation to the space in the S-VOL from where the data is not yet
copied, or a request for read/write operation to the space in the T-VOL to where the data is not
yet copied, Compatible FlashCopy first copies the old data in the S-VOL to the T-VOL. This process
is called on-demand copying. The following figure shows the process of the on-demand copying.
Figure 3 On-demand copying
1. From the host, there is a write request to the space in the 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 the on-demand copying process, Compatible FlashCopy does not automatically
withdraw the relationship. Therefore, when you select NOCOPY mode, you must withdraw the
pair by using the FCWITHDR command. If you select the NOCOPY2COPY mode for the relationship
made by NOCOPY mode, the background copying process is performed. When all the data is
copied, the relationship is released automatically.
8 About Compatible FlashCopy operations