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58 IBM System Storage DS6000 Series: Copy Services with IBM System z
All three bitmaps are necessary for incremental FlashCopy:
Target bitmap - This bitmap keeps track of tracks not yet copied from source to target.
Source Change Recording bitmap - This bitmap keeps track of changes to the source.
Target Change Recording bitmap - Yhis bitmap keeps track of changes to the target.
These bitmaps allow subsequent FlashCopies to transmit only those blocks of data for which
updates occurred. Every write operation to the source or target volume will be reflected in
these bitmaps by setting the corresponding bit to 0.
When the
Refresh takes place, the bitmap used for change recording is used to analyze
which blocks need to be copied from the source volume to the target volume (see Figure 7-5)
.
Figure 7-5 Refresh of the FlashCopy target volume
After the Refresh—which takes place only on the bitmap level—the new FlashCopy based on
time-0’ is active. The copy of the time-0’ data to the target is done in the background.
Tip: You can do the incremental copy at any time. You do not have to wait for the previous
background copy to complete.
time
Refresh FlashCopy target volume
t
0
0 0 1 000
bitmap
data
target
data
t
x
t
y
needs to be copied as a
write occured on the target
t
z
0 0 1 100
a
b
t
t
bitmap
c
t
source
t t t ttt
00z
t t t t tt
x
0
0
xz0
0
0
0
update to the source
needs to be copied
update to the source
needs to be copied
needs to be copied as a
write occured on the target
1 1 1 111
bitmap
data
target
data
1 1 1 111
source
t t t ttt
0' 0' 0'
0'
0'
0'
t t
0'
0'
t
0'