IBM Storwize V7000 Unified Problem Determination Manual page 328

pool that is used for that file system. The Capacity column displays the current
used capacity for the file and the underlying block storage pools. To view specific
thresholds for individual volumes, select the NSDs filter to display the block
volumes that are used in the file system. To view specific thresholds for individual
volumes, right-click a volume and select Properties. In the upper right of the
Properties panel, an allocation bar is displayed with the current threshold indicated
by a red vertical bar.
Whenever a threshold is reached and an alert is issued, the system suggests actions
that correspond to the specific scenario. If action is not taken and the storage pool
reaches 100% utilization, volumes and their related network shared disks (NSDs)
can go offline, which causes the file system to go offline. To see an overview of
recovery scenarios, go to Chapter 8, "Troubleshooting compressed file systems," on
page 297.
Theoretically, the total virtual capacity for all volumes in a pool can exceed the
actual physical capacity that is available to the storage pool. For example, an
administrator creates a 10 TB file system from a storage pool that has 10 TB of
capacity. In this example, one volume is used and is allocated the full 10 TB of
capacity to store this data. On average, the data that is stored in this file system
has 60% compression savings. After the file system is full with 10 TB of data that
gets 60% compression savings, it has actually used only 4 TB of physical capacity
from the pool to store the compressed data. To use the remaining 6 TB of unused
capacity, virtual capacity can be added for the volumes in the pool.
However, in reality, you need contingency capacity on the storage pool that
remains unallocated and available to minimize impact to capacity utilization when
data changes affects compression rates. In most cases, data does not have the same
compression rate because it is constantly changing over the course of life cycle.
Incompressible data or data that does not compress well can be added to a file
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)
Physical capacity in TB (PC)
Compression savings (CR)
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
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Storwize V7000 Unified: Problem Determination Guide Version
0.8 to represent 80% contingency threshold.
10 TB physical capacity that is available in the pool.
0.6 represents 60% compression savings.