IBM System storage DS6000 Series Redbook page 71

Reading from the source
The data is read immediately (see Figure 6-3 on page 46).
Writing to the source
Whenever data is written to the source volume while the FlashCopy relationship exists,
the storage subsystem makes sure that the time-zero-data is copied to the target volume
prior to overwriting it in the source volume.
To identify if the data of the physical block on the source volume needs to be copied to the
target volume, the bitmap is analyzed. If it identifies that the time-zero data is not available
on the target volume, then the data will be copied from source to target. If it states that the
time-zero data has already been copied to the target volume then no further action is done
(see Figure 6-3).
It is possible to use the target volume immediately—for reading data and also for writing
data.
Reading from the target
Whenever a read-request goes to the target while the FlashCopy relationship exists, the
bitmap is used to identify if the data has to be retrieved from the source or from the target.
If the bitmap states that the time-zero data hasn't yet been copied to the target, then the
physical read is directed to the source. If the time-zero data has already been copied to
the target then the read will be performed immediately against the target (see Figure 6-3).
Writing to the target
Whenever data is written to the target volume while the FlashCopy relationship exists, the
storage subsystem makes sure that the bitmap is updated. This way the time-zero data
from the source volume never overwrites updates done directly to the target volume (see
Figure 6-4).
Writing to the target
t
0
source
t t t t t
0 x 0 y
0
Figure 6-4 Writes to target volume
Terminating the FlashCopy relationship
The FlashCopy relationship is
the source volume to the target volume. The relationship can also be
by issuing the corresponding commands. If the
then the FlashCopy relationship must be withdrawn explicitly.
t
a
write a
0 0
data
t t t
0 0
a
automatically ended
persistent
time
t
b
write b
bitmap
1 0 1 1
target
data
t
b
when all tracks have been copied from
explicitly withdrawn
FlashCopy option was specified
Chapter 6. FlashCopy overview
47