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336 IBM System Storage DS6000 Series: Copy Services with IBM System z
25.1 Performance aspects for Global Mirror
Global Mirror is basically comprised of Global Copy and FlashCopy, and it combines both
functions to create a solution that provides consistent data at a distant site. Global Copy has,
at most, only a minimal impact on the response time of an application write I/O to the Global
Copy primary volumes.
In the Global Mirror environment, FlashCopy is used with the
nocopy attribute. This implies
that, for any write I/Os to the FlashCopy source volume, there will be additional internally
triggered I/Os in the remote storage disk subsystem. These I/Os preserve the FlashCopy
source volume tracks by making a copy to the FlashCopy target volume
before the source
tracks are updated. This happens every time within the interval in between two FlashCopy
establish operations.
Note that the nocopy attribute does not start a background copy operation from source to
target. It only maintains a set of FlashCopy bitmaps for the source and target volumes. These
bitmaps are established the first time that the FlashCopy relationship is created with the
nocopy attribute. Before a source track is modified between the creation of two Consistency
Groups, the track is copied to the target volume to preserve the previous point-in-time copy.
This includes updates to the corresponding bitmaps to reflect the new location of the track,
which belongs to the point-in-time copy. Be aware that each Global Copy write to a secondary
volume in the window of two adjacent Consistency Groups causes such a FlashCopy I/O
operation.
For an application I/O within this context, there is also a Global Copy I/O when Global Copy
replicates a track from the primary volume to the secondary volume. This implies an
additional read I/O in the primary storage disk subsystem and a corresponding write I/O to the
secondary storage disk subsystem. In a Global Mirror environment, the Global Copy
secondary volume is also the FlashCopy source volume. This might have some effect in a
very busy disk subsystem.
Figure 25-1 approximates what happens in between two Consistency Group creation events.
The application write I/O completes immediately at the local site (1). Global Copy reads the
modified track at the local site (2). Before the track gets updated on the B volume, the
FlashCopy source track is preserved on the C volume because of the nocopy attribute. This is
a read (3) and a write (4) which are done to preserve the source track writing it to the C
volume. Finally, the track is updated on the B volume by another write (5). This is not
necessarily the exact sequence of internal I/O events, but rather an approximation. There are
optimization and consolidation effects that make the entire process more efficient.
Figure 25-1 Application write I/O within two Consistency Group formation events
Primary
Primary
A
Primary
PENDING
Write
Host
FICON
A1
Read
FCP links
Primary
Primary
A
Secondary
PENDING
B1
Primary
Primary
Tertiary
C1
Write
Read
Write
1
2
3
4
5
Remote site
Local site