Key capacity concepts

Understand the terms that are used to measure and plan capacity.

Scenarios are used to explore the following capacity concepts for block storage systems:

Standard provisioning
Thin-provisioning

Tip: To review the capacity terms and definitions that are used, click Definitions for the key capacity terms and concepts .

Standard provisioning

You add Storage System A with a usable capacity of 100 TiB to IBM Storage Insights for monitoring.

Then, you add a standard-total provisioned with a capacity of 20 TiB.

You create standard-provisioned volumes when you want to dedicate the usable capacity of the storage system to the device that writes to it. From the perspective of the pool and storage system, the capacity is used and is no longer available to the storage system and pool.

The capacity values that are reported by the storage system for standard-provisioned volumes are as follows:

Used Capacity (%) is reported as fully allocated, and Used Capacity (GiB) has the same value as the total provisioned of the volume.
Available Capacity (%) is reported as 00.00.

Thin-provisioning

Unlike standard-provisioned volumes, the capacity of thin-provisioned volumes is not dedicated to the device that writes to it. Devices get the capacity when they write data to the thin-provisioned volumes. Depending on the data reduction techniques that are supported by the storage system, the data that is written to the thin-provisioned volumes can be reduced before it is stored on the volumes.

Thin-provisioning savings

Thin-provisioning savings are the total amount of capacity that is saved in a pool, system, or volume by using capacity when needed as a result of write operations. The capacity that is saved is the difference between the total provisioned and the written capacity.

In this scenario, you add four thin-provisioned volumes with a total provisioned of 200 TiB to Storage System B, which has a usable capacity of 100 TiB.

Thin-provisioning savings are the difference between the 200 TiB total provisioned to the thin-provisioned volumes and the 150 TiB of data that is written to the volumes, which is 50 TiB.

Data reduction savings

Data reduction savings are the total amount of usable capacity that is saved in a pool, system, or volume through the application of data reduction algorithms on the written data, such as compression and deduplication. This saved capacity is the difference between the written capacity and the used capacity.

Before data is stored on thin-provisioned volumes, it can be reduced. For example, if your storage system supports compression and deduplication, these data reduction techniques are applied to reduce the amount of capacity that is needed to store the data.

In this scenario, the data reduction savings are the difference between the written capacity (150 TiB), and the used capacity(50 TiB), which is 100 TiB or 67%.

Total Savings

Total savings are the amount of capacity that is saved in a pool, system, or volume through thin-provisioning and data reduction techniques.

In the scenario illustrated above, the thin-provisioning savings are (200 TiB - 150 TiB), which is 50 TiB. The data reductions savings are (150 TiB - 50 TiB), which is 100 TiB. So, the total capacity savings are 150 TiB.

Effective capacity

Effective capacity is the amount of total provisioned capacity that can be created in a system or pool without running out of capacity given the current data reduction savings that are being achieved. To calculate effective capacity, you divide the capacity of the storage system or pool by the data reduction savings percentage.

Note: In some storage systems, restrictions in the system determine the maximum total provisioned that is allowed in a pool or system. In these cases, the effective capacity cannot exceed this limit.

You can use effective capacity to estimate how much capacity you can provision to your thin-provisioned volumes without running out of usable capacity.

Provisioning based on effective capacity

In the scenario illustrated above, the storage system has a usable capacity of 100 TiB and a current data reduction savings of 66%. So, the effective capacity is 100/(1.00 - 0.67), which is 303 TiB. Currently, the total provisioned of the volumes is 200 TiB. Assuming that the same data reduction savings are achieved for subsequent write operations, more thin-provisioned volumes can be created with a total capacity of 303 TiB.

Definitions for the key capacity terms and concepts

These are capacity terms and concepts that are used:

Available capacity: The amount of usable capacity that is not yet used in a system, pool, array, or MDisk.
Compression: A function that removes repetitive characters, spaces, strings of characters, or binary data from the data being processed and replaces characters with control characters. Compression reduces the amount of storage space that is required for data.
Data deduplication: A method of reducing storage needs by eliminating redundant data. Only one instance of the data is retained on storage media. Other instances of the same data are replaced with a pointer to the retained instance.
Data reduction: A set of techniques that can be used to reduce the amount of usable capacity that is required to store data. Examples of data reduction include data deduplication and compression.
Data reduction savings: The total amount of usable capacity that is saved in a pool, system, or volume through the application of data reduction algorithms on the written data, such as compression and deduplication. This saved capacity is the difference between the written capacity and the used capacity.
Effective capacity: The amount of total provisioned capacity that can be created in a system or pool without running out of usable capacity given the current data reduction savings that are being achieved. This capacity equals the usable capacity that is divided by the data reduction savings percentage. In some storage systems, restrictions in the system determine the maximum provisioned capacity that is allowed in a pool or system. In those cases, the effective capacity cannot exceed this limit.
Total Provisioned: Total capacity of all volumes that are created on the system, which includes volumes that are not mapped to hosts.
Standard-provisioned volumes: Unlike thin-provisioned volumes, which use capacity when it is needed, the capacity that is provisioned to standard-provisioned or thick volumes is no longer available to the pool. Standard-provisioned volumes are fully allocated, and their capacity is reported as used because their capacity is no longer available to the pool and storage system.
Thin-provisioning: The ability to defer capacity allocation on a storage resource until data is actually written to it.
Thin-provisioning savings: The total amount of usable capacity that is saved in a pool, system, or volume by using usable capacity when needed as a result of write operations. The capacity that is saved is the difference between the total provisioned minus the written capacity.
Total savings: The total amount of usable capacity that is saved in a pool, system, or volume through thin-provisioning and data reduction techniques. This saved capacity is the difference between the used capacity and the total provisioned.
Usable Capacity: The amount of usable capacity that is available for storing data on a system, pool, array, or managed disk after formatting and RAID techniques are applied.
Used capacity: The amount of capacity that is taken up by data in a system, pool, array, or MDisk after data reduction techniques have been applied.
Written capacity: The amount of usable capacity that would have been used to store written data in a pool or system if data reduction was not applied.