In this section system resource usage refers to the two main components of the system relative to data reduction, capacity and cache.
Capacity is displayed as subscribed, and usable. Subscribed capacity represents the amount of capacity presented to hosts or applications. Subscribed capacity exceeding the total usable capacity is commonly referred to as subscription or virtual provisioning. This allows users to present more capacity to applications or hosts than the system can store. This is displayed in Unisphere as the Subscribed Usable Capacity Percent. Usable capacity is the amount of disk capacity available to store application data. The total amount of usable capacity is determined by the capacity of the physical disks that are configured in the system.
Like previous generations, PowerMax operates as a cache centric architecture. All data is passed through cache before being stored on disk. It is used to support multiple functions within the system not simply host I/O. Provisioning, local replication and data reduction also use cache. Cache is divided into two main sections, data cache and metadata cache. (see Figure 4). Cache usage is displayed in the Unisphere capacity report under the System usage section, represented as metadata used (see Figure 6).
- Data cache: Represents the amount of cache available for host IO, reads and writes from hosts or applications. The system configuration ensures there is always data cache available for host I/O.
- Metadata cache: This is comprised of three sections, Front-End, Replication and Back-End. Each section represents an amount of metadata cache it can consume.
- Front-End Metadata: At initial install the percentage used will show zero as there is no subscribed capacity. There are two factors that will cause front-end metadata usage to increase, provisioning capacity to hosts or applications as well as host allocations. In PowerMax systems the increase is primarily due to host allocations.
- Replication Metadata: At initial install the percentage used will show zero as there is no local replication activity. As local replication is used the percentage will increase up to 100 percent. When replication metadata has reached 100 percent, local replication has reached its limit. (for more information refer to dell-emc-powermax-vmax-all-flash-timefinder-snapvx-local-replication.pdf)
- Back-End Metadata: At initial install the percentage used represent the initial layout of compression pools. As the compression pools are expanded to support more effective capacity the usage can grow up to 100%. When Back-End Metadata shows 100% used it indicates the system has expanded usable capacity to the maximum effective capacity the system can support. This has no impact on Front end metadata growth or the ability to support Host I/O.
Figure 4. Cache breakdown
Array usage:
- Subscribed Capacity: The bar presented represents the total provisioned capacity. The dark shaded portion of the bar indicates host allocations of the presented capacity.
- Snapshot Capacity: The total represents the sum of all existing snapshots. The dark shaded portion of the bar represents the amount of existing snapshot capacity that has been modified. The modified capacity also represents additional usable capacity that is consumed by snapshot data.
- Usable Capacity: The total amount of usable disk space available. The dark shaded portion represents the amount of disk space that is consumed.
- Subscribed Usable Capacity: The percent displayed represents the amount of subscribed capacity in relation to the total amount of usable capacity.
Figure 5. Array usage from the Unisphere for PowerMax Capacity report
System Usage: The capacity report in Unisphere for PowerMax displays meta data usage in the form of percentage used. The values displayed represent the amount of metadata used for each function. These values are also available in Solution Enabler and REST API.
Figure 6. System Usage as seen in Unisphere for PowerMax
- System Metadata: Represents the total metadata usage for the system. The amount of cache used by the system for all functions supported by metadata. The system used percentage represents the usage encompassing the total amount of metadata cache available.
- Replication Metadata: A cache resource used in the form of metadata to support replication data pointers used with local replication. At initial install the percent used starts at zero as there is no local replication activity. The total amount of cache available for replication metadata is based on the configuration of the system and will not increase with the use of local replication. When replication metadata has reached its maximum the use of local replication has reached its limit.
- Front-End Metadata: A cache resource used in the form of metadata to support subscribed capacity and host allocations. As subscribed capacity is increased, the amount of front-end metadata increases. In VMAX All Flash systems provisioning will cause this to increase at the time of device creation. In PowerMax system the increase is primarily due to host allocations. In both cases the increase of Front-End metadata can consume data cache.