HP XP7 Storage - Manual (Page 38)

2. Calculate the number of bitmap areas per volume.
In the following calculation, differential data is measured in bits. 122,752 bits is the amount
of differential data per bitmap area.
Number of bitmap areas = ceil ( (Number of cylinders x 15) / 122,752)
NOTE: Doing this calculation for multiple volumes can result in inaccuracies. Perform the
calculation for each volume seperately, then total the bitmap areas. The following examples
show correct and incorrect calculations. Two volumes are used: one of 10,017 cylinders and
another of 32,760 cylinders
Correct calculation
ceil ((10,017 x 15) / 122,752) = 2
ceil ((32,760 x 15) / 122,752) = 5
Total: 7
Incorrect calculation
10,017 + 32,760 = 42,777 cylinders
ceil ((42,777 x 15) / 122,752) = 6
3. Calculate the maximum number of pairs, which is restricted by the following:
•
The number of bitmap areas required for Continuous Access Synchronous Z (calculated
above).
•
The total number of bitmap areas in the storage system, which is 65,536.
Bitmap areas are also used by Continuous Access Synchronous, Continuous Access
Journal, and Continuous Access Journal Z.
Therefore, the number of bitmap areas used by these other program products (if any)
must be subtracted from 65,536, with the difference used to calculate the maximum
number of pairs for Continuous Access Synchronous Z.
If Continuous Access Synchronous Z and Continuous Access Journal Z share the same
volume, you must use the total number of bitmap areas for both pairs regardless of whether
the shared volume is local or remote.
•
The maximum number of pairs supported per storage system, which is 65,280 (65,279
if BCM is used).
Use the following formula:
Maximum number of pairs = floor (Total number of bitmap areas in
the storage system / Required number of bitmap areas)
If the calculated maximum number of pairs exceeds the total number of LDEVs of the storage
system, and the total number of LDEVs of the storage system is less than 65,280, then the total
number of LDEVs of the storage system becomes the maximum number of pairs.
Priority for initial copy operations and scheduling order
Though you can specify a maximum number of initial copies to run concurrently, you are allowed
to perform more initial copies than this when creating pairs. If you do this, you will set the scheduling
order for the additional operations in the Priority field when creating the pair.
This topic provides two examples in which priority and the pre-set scheduling order are discussed.
NOTE: The Initial Copy Priority option discussed here is not supported by the following non-RWC
commands: PPRC CESTPAIR, BCM YKMAKE, and RAID Manager paircreate. When the pair is
created using these commands, the initial copy is performed according to the order that the
command is run.
38 Planning for Continuous Access Synchronous Z