Compressed volumes

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When you create volumes, you can specify compression as a method to save capacity for the volume. With compressed volumes, data is compressed as it is written to disk, saving more space. When data is read to hosts, the data is decompressed.

Compression is available through data reduction support as part of the system. If you want volumes to use compression as part of data reduction support, compressed volumes must belong to data reduction pools. Data reduction pools also support reclaiming unused capacity automatically after mapped hosts no longer need the capacity for operations. These hosts issue SCSI unmap commands and the released capacity is reclaimed by the data reduction pool for redistribution. For compressed volumes in data reduction pools, the used capacity before compression indicates the total amount of data that is written to volume copies in the storage pool before data reduction occurs. If your system uses multiple technologies to compress data you can also have compressed volumes in standard pools, but these pools do not support reclaiming unused capacity. If your system uses multiple technologies to compress data, you can also have compressed volumes in standard pools, but these pools do not support reclaiming unused capacity. If you have existing compressed volumes in standard pools, the following values help determine capacity for each compressed volume:

Real capacity: Indicates the extent capacity that is reserved from the standard pool. The real capacity is set when the compressed volume is created and can be expanded or shrunk down to the used capacity.
Provisioned capacity: Indicates the capacity that is available to hosts. The provisioned capacity is set when the compressed volume is created and can be expanded or shrunk afterward.
Used capacity: Indicates the amount of real capacity that is used to store customer data and metadata after compression.

You can also monitor information on compression usage to determine the savings to your storage capacity when volumes are compressed. To monitor system-wide compression savings and capacity, select Dashboard to view the Capacity Saving. You can compare the amount of capacity that is used before compression is applied to the capacity that is used for all compressed volumes. In addition, you can view the total percentage of capacity savings when compression is used on the system. For systems that use multiple compression technologies, such as data reduction pools or self-compressing drives, the total percentage of capacity savings cannot be determined at a system level. To view each of the capacity savings for all the supported compression technologies for the system, select View Compression Details. You can also monitor compression savings across individual pools. You can also monitor compression savings across individual pools.

If your system currently does not use compression, the system automatically analyzes your configuration to determine the potential storage savings if compression is used. The management GUI incorporates the Comprestimator utility that uses mathematical and statistical algorithms to create potential compression savings for the system. The analysis for potential savings can be used to determine whether purchasing a compression license for the system is necessary to reduce cost of extra storage devices. To estimate compression savings in the management GUI, select Volumes > Actions > Space Savings > Estimate Compression Savings. For example, you can run the analyzevdisk command on a single volume. You can also use the analyzevdiskbysystem command to analyze all of the volumes that are on the system. Any volumes that are created after the compression analysis completes can be evaluated individually for compression savings. Ensure that volumes to be analyzed contain as much active data as possible rather than volumes that are mostly empty of data. Analyzing active data increases accuracy and reduces the risk of analyzing old data that is already deleted but can still have traces on the device.

After the analysis completes, you can download a savings report that shows estimated savings for all the volumes with enough data to be analyzed. This report lists all currently configured volumes on the system and their potential compressions savings. To download a report, select Volumes > Volumes > Actions > Capacity Savings > Download Savings Report. You can also display the results by using the lsvdiskanalysis command. You can display results for all the volumes or single volumes by specifying a volume name or identifier for individual analysis.

Size limits

If you are using compressed volumes in standard pools, these volumes have the following size limits. If a new or existing compressed volume in a standard pool approaches the maximum size, the system issues an alert. Compressed volumes in data reduction pools do not monitor size of the volumes.

96 TB: Maximum virtual size of a new, individual compressed volume. You cannot create a new compressed volume that exceeds this size. In addition, you cannot increase the size of an existing compressed volume beyond this value. If one or more compressed volumes in a system exceed this limit, you receive an alert. To reduce the risk of losing or corrupting data, you must take action soon to remove data from the compressed volume.
120 TB: Maximum virtual size of an existing compressed volume in a system. If any compressed volumes in the system approach or exceed this value, the system issues an alert.
Important: Immediate action is required to remove all data from the compressed volume and prevent the loss of data.
128 TB: Maximum physical size of a compressed volume.

For information about how to move data off a compressed volume in a standard pool, view the topic on the IBM Support portal website for your product. Search for your product, then select the Flashes, alerts and bulletins link under Documents on the support page for your product.