Chapter 3. Planning For Compression; Understanding Compression Rates, Ratios, And Savings; Prerequisites And Limitations - IBM XIV Gen3 Series Planning Manual

Chapter 3. Planning for compression

Before implementing IBM Real-time Compression on your system, assess the
current types of data and volumes that are used on your system.
Note: If you are using IBM XIV Storage System software version 11.6.x or later,
and IBM XIV Management Tools version 4.6 or later, you can see storage saving
estimates (even if compression is not licensed or is just disabled). The decision to
compress volumes can be based on the expected storage savings of the compressed
data and the expected effect on performance.

Understanding compression rates, ratios, and savings

Consider a use case where the original capacity required to hold data was 100 TB,
but 20 TB after compression (100 TB = 20 TB + 80 TB saved).
The following values help to clarify these terms:
Compression rate = 80%
Compression savings rate = 80%
Compression savings = 80 TB
Compression ratio = original size (100 TB) divided by the size on disk after
compression (20 TB) = 5:1
When you consider savings, it is easiest to use the compression rate.
The compression ratio helps in understanding how much effective data you can
store on your system. So, when you have a 5:1 compression ratio, you will be able
to store 500 TB of data on 100 TB of physical capacity.

Prerequisites and limitations

The following prerequisites and limitations are for XIV Storage System models 114
or 214, with XIV Storage software version 11.6 or later, and models 214 or 314 with
XIV Storage software version 11.6.1 or later.
Tip: Consider using version 11.6.1 with model 214 to benefit from improvements
like compressed volume size.
v Compressed volumes must be created in thin-provisioned pools.
v To convert a mirrored volume from uncompressed to compressed (or vice versa),
v Snapshots cannot be converted from compressed to uncompressed and vice
v Space requirements:
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the mirroring relationship for that volume must first be removed and then
recreated after the conversion.
versa. Snapshots that already existed before a volume was converted from
non-compressed to compressed are not converted and are not available with the
converted volume.
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