Performance Aspects For Global Mirror - IBM System storage DS6000 Series Redbook

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
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
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.
1
Write
FICON
Host
Figure 25-1 Application write I/O within two Consistency Group formation events
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IBM System Storage DS6000 Series: Copy Services with IBM System z
A
2
Read
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Local site
nocopy
attribute. This implies
before
5
A
Write
Write
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the source
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Tertiary