Physical volume planning

Use this section to determine the number of physical volumes that are required to accommodate the workload you are planning for the TS7740, the TS7720 tape attach, or the TS7760 tape attach.

To determine the number of physical volumes that are required to accommodate your workload, consider:

Amount of data stored for a given host workload
Average compression ratio achieved per workload
Average utilization rate of filling physical volumes
Scratch physical volumes

Amount of data stored for a given host workload

The amount of data that is stored per workload can be extracted from your Tape Management System, such as RMM, or from TS7700 by using VEHSTATS.

Average compression ratio achieved per workload

The data that a host writes to a virtual volume might be compressible. The space that is required on a physical volume is calculated after the effect of compression. If you do not know the average number for your data, assume a conservative 2:1 ratio.

Average utilization rate of filling physical volumes

The average utilization rate of filling physical volumes can be calculated from the Reclaim Threshold Percentage. The percentage that is used to determine when to perform reclamation of free storage on a stacked volume. When the amount of active data on a physical stacked volume drops below this percentage, a reclaim operation is performed on the stacked volume. The valid range of possible values is 0 - 95%; 35% is the default value. Therefore, the utilization rate of filling physical volumes should range from 35% to 100%. The average utilization rate of filling physical volumes can be calculated as (35+100)/2 = 67.5%.

Scratch physical volumes

You should refer the following diagram to determine the number of scratch physical volumes you need for each pool.

Is the E08 or E07 drive installed?
- Yes
  1. Is borrow/return sharing enabled?
    - Yes: 15 volumes in the common scratch pool
    - No: 15 volumes in each dedicated pool
- No
  1. Is borrow/return sharing enabled?
    - Yes: 50 volumes in the common scratch pool
    - No: 50 volumes in each dedicated pool

If the number of scratch physical volumes in your system is fewer than these thresholds:

Reclamation of sunset media does not occur
Reclamation runs more frequently

You can have less than 15 or 50 volumes in your pool if these conditions are acceptable. Keep in mind that you need at least 2 scratch physical volumes to avoid an out of scratch state.

The following is a suggested formula to calculate the number of physical volumes needed:

Where:

Pv =	Total number of physical volumes needed
Da =	Total amount of data that is returned from your Tape Management System or VEHSTATS per workload
Cr =	Compression Ratio per workload
	(Use Cr=1 when Da represents previously compressed data)
Ut =	Average utilization rate of filling physical volumes
Pc =	Capacity of a physical volume in TB
Px =	Resulting number of physical volumes needed for a particular workload "x"
Ps =	Number of physical volumes in common scratch pool

For each workload, calculate the number of physical volumes needed:
Px = (Da/Cr)/(Pc × Ut/100)

Next, add in physical scratch counts and the Px results from all known workloads:
Pv = Ps + P1 + P2 + P3 + ...

For example, by using the following assumptions:

Workload 1

Da
Cr
Ut
Pc

=
=
=
=

100 TB
2
67.5%
10 TB (capacity of a JD volume)

P1 = (100/2)/(10 × 67.5/100) = 8 physical volumes

Workload 2

Da
Cr
Ut
Pc

=
=
=
=

150 TB
2
67.5%
7 TB (capacity of a JC volume in 3592-E08 format)

P2 = (150/2)/(7 × 67.5/100) = 16 physical volumes

If the number of physical volumes in the common scratch pool is Ps = 15, you would need to plan on the following number of physical volumes in the TS7740, the TS7720 tape attach, or the TS7760 tape attach:

Pv = Ps + P1 + P2 = 15 + 8 + 16 = 39 physical volumes

If you need dual copied virtual volumes, you need to double the number of physical volumes for that workload. If a workload uses dedicated pools with the borrow/return sharing disabled, then each workload must have its own dedicated additional scratch count versus the shared Ps count.