IBM Storwize V7000 Unified Problem Determination Manual page 461

system, which impacts compression rates. The system default for the contingency
threshold at 80% of the physical capacity which provides 20% contingency capacity
for the storage pool, which is adequate for most environment. For example, if an
administrator has a storage pool with 10 TB of physical storage and sets the
threshold to 80%, only 8 TB out of the physical 10 TB are available in the pool.
However, if the data in the pool receives 60% compression savings, the
administrator can store approximately 20 TB of uncompressed user data in 8 TB of
physical space. In this way, the maximum amount of virtual capacity exceeds the
physical capacity for the compressed storage pool. To calculate the recommended
virtual capacity, you can use the following equation:
Recommended maximum virtual capacity (in TB) = (CT * PC) * (1 / (1 – CR))
Contingency threshold (CT)
0.8 to represent 80% contingency threshold.
Physical capacity in TB (PC)
10 TB physical capacity that is available in the pool.
Compression savings (CR)
0.6 represents 60% compression savings.
File System Capacity Management
Additionally you must also monitor file capacity utilization to ensure that the file
system does not reach 100% utilization and run out of capacity. The capacity
utilization of a file system issued physical capacity in the compressed pool. The
system uses the same threshold and alerting system and suggests corrective actions
when thresholds are reached. If based on the original, uncompressed capacity that
the system presents to users and applications of the file system.
To free up capacity in a file system, you can either remove NSDs from the file
system or increase the current capacity of the storage pool, which can be used to
expand the volumes that are related to the NSDs from the unused physical
capacity. If corrective action to reduce utilization is not performed before the file
system reaches 100% utilization, the file system goes offline and no longer handles
read and write requests.
Chapter 8. Troubleshooting compressed file systems
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