Hewlett Packard Enterprise HPE XP7, User Manual, Page: 66 / 311

Fast Snap cache management device requirements on page 68
Calculating the number of cache management devices
You can calculate the number of cache management devices you must reserve to initially create a Fast
Snap pair for a volume.
8,192 cache management devices are required to create FS pools in a system for the first time. For
details about the number of cache management devices required for creating THP pools, see the
Provisioning Guide . Fast Snap must reserve 4,096 cache management devices out of 8,192.
When you create Fast Snap pairs in volumes for the first time, confirm that at least 4,097 cache
management devices are reserved. If the number is less than 4,097, pairs might not be created.
Use the following formula:
Number of cache management devices that you must reserve =
ceil (Size of P-VOL [TB] / 2.6)
If the amount of pool usage for the P-VOL exceeds 70 percent of the total capacity of the cache
management devices reserved for the P-VOL, you must reserve another cache management device. You
can reserve a maximum of 256 cache management devices for each P-VOL.
More information
Calculating the number of Fast Snap pairs based on pair tables on page 64
Calculating Fast Snap pairs based on the snapshot estimated manageable capacity on page 64
Calculating Fast Snap pairs based on cache management devices on page 65
Calculating the Fast Snap pair capacity for THP pools on page 66
Calculating and assigning pool capacity on page 69
External Storage and external volumes used as pool-VOLs on page 71
Simultaneous processing of multiple Fast Snap pair tasks on page 71
Pair operations when pairs are cascaded on page 72
Pair operations when pairs are not cascaded on page 83
Pair operations when a P-VOL is shared by multiple S-VOLs on page 84
Data recovery and backup differences between Fast Snap and Business Copy on page 85
Fast Snap cache management device requirements on page 68
Calculating the Fast Snap pair capacity for THP pools
When creating a THP pool, you can set the maximum reserved V-VOL capacity against the pool capacity.
For details about the maximum reserved capacity, see the
HPE XP7 Provisioning for Open Systems User
Guide .
When storing snapshot data in a THP pool, set the maximum reserved V-VOL capacity against the THP
pool by calculating the V-VOL capacity of a Fast Snap pair (Fast Snap pair capacity) using the following
formula.
Fast Snap pair capacity = Σ↑(P-VOL capacity (MB)
× Number of pairs × 2 ÷ 42 (MB))↑ × 42 (MB) + Σ↑(P-VOL capacity (MB) × Number of pairs
× 2 ÷ 2,921,688 (MB))↑ × 175,434 (MB)
Decimal places of the value enclosed by arrows (↑ value ↑) are rounded up.
As for Number of pairs in the formula, allocated pages are not released if you delete pairs. Because of
this, use the maximum number of FS pairs you want to create in the applicable snapshot tree as Number
of pairs instead of using the number of pairs currently created in the applicable snapshot tree.
66 Calculating the number of cache management devices