Monitoring usable capacity

If you are using the command-line interface to determine usable capacity on your system, several parameter values are used from the lssystem command to calculate stored, available, and total capacities. Stored capacity is calculated with the values in the total_mdisk_capacity, total_free_space, total_reclaimable_capacity by using the following formula:

• Total stored capacity = total_mdisk_capacity - total_free_space - total_reclaimable_capacity

To calculate the available capacity, use the values in total_free_space and total_reclaimable_capacity in the following formula:

• Total available capacity = total_free_space + total_reclaimable_capacity

The value in the total_mdisk_capacity in the lssystem command indicates the total usable capacity is on the system.

The system supports virtualizing capacity that is on external storage systems that are attached to the system. For external storage systems, administrators must configure out-of-space alerts that notify the system when usable capacity on the external storage system reaches a defined threshold. Without these thresholds defined on the external storage systems, external storage on these systems can become overprovisioned and risk running out of usable capacity that is used for host operations. Overprovisioned external storage occurs when the sum of the provisioned capacity of all the volumes in a system or pool is greater than the total usable capacity of the system or pool that is allocated from the external storage system. The system detects potential overprovisioned external storage and presents a warning on the Dashboard page. If this warning is displayed, select Overprovisioned External Storage to display pools that contain storage that is nearing these overprovisioned scenarios. Depending on how external storage is configured in pools, different strategies and best practices can be used to ensure that usable capacity is sufficient and reported accurately to the system.

Configuration Scenario 1: One pool contains storage from one external storage system

In this configuration, storage administrators have the same concern of running out of capacity as when data is written directly to external storage systems. Storage administrators must set up out-of-space alerts on external storage and monitor usage to ensure that the I/O operations do not exceed the usable capacity. Consult documentation for your external storage system for specific guidelines on these thresholds. If an out-of-space condition occurs, then usable capacity must be freed by deleting data or volumes.

Configuration Scenario 2: Multiple pools contain a single tier of storage from multiple external storage systems

In this configuration, multiple pools use the same type of storage, or tier, across several external storage systems. If capacity from different external storage systems is shared across multiple pools, then provisioning groups are created. Provisioning groups are objects that identify whether storage is shared across multiple pools. The MDisks by Pools page in the management GUI, displays all pools and their assigned MDisks. If you are not sure whether external storage systems are shared across several pools, right-click the pool and select View Resources to display the provisioning group that is associated with the pool. In this configuration, the system spreads extent allocations across all the external storage within the provisioning group to ensure that space is used evenly across all the external storage systems. However, usable capacity on external storage systems within the provisioning groups can still become overprovisioned, so storage administrators need to configure out-of-space alerts and monitor usable capacity to determine how capacity is being used on the system. If low-space warning occurs on virtualized MDisks on external storage and usable capacity is available on other external storage system in the pool, you can remove some of these MDisks from the pool until the usable capacity is within reasonable limits. This process migrates data to other MDisks in the pool. Ensure that enough usable capacity is available on the other external storage systems in the pool so that they do not run out of space during this operation.

Configuration Scenario 3: Multiple pools contain different tiers of storage from multiple external storage systems

In this configuration, different tiers of storage are present on the external storage systems within the pools. As with the previous configuration, provisioning groups are created to identify the external storage that is shared within the pools. However, in this case the system attempts to allocate the entire usable capacity in the top tier rather than spreading extents across provisioning groups. Storage administrators need to monitor the usable capacity in these top tiers of storage to ensure that space is sufficient for the workloads. If low-space warnings occur on virtualized MDisks on external storage and usable capacity is available on other external storage system in the pool, you can remove some of these MDisks from the pool until the usable capacity is within reasonable limits. This process moves data to other MDisks in the pool. Ensure that enough usable capacity is available on the other external storage systems in the pool so that they do not run out of space during this operation.

Configuration Scenario 4: Data reduction pools

Systems that support data reduction pools, which use data reduction technologies, like compression, can configure these types of pools to minimize overallocation on the external storage system. Data reduction pools also support the ability to reclaim unused capacity from host unmap operations and volume deletions. In the management GUI, select Pools > MDisks by Pools to view all the pools configured on the system. Data reduction pools are shown with Data Reduction set to Yes. When you use data reduction pools that virtualize external storage that can run out of space, always enable compression when you create volumes in these pools. The external storage system must be configured to present only usable capacity to the system, as the size of the written data cannot be reduced any further.

To assist with storage planning, Table 1 lists the low space warnings that can be generated by the system for compressed arrays. Error 1246 is not raised on Easy Tier® pools or data reduction pools.