Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax 2500 and 8500: TimeFinder SnapVX Snapshots and Clones > Snapshot capacity
Host writes to source volumes create snapshot deltas in the Storage Resource Pool (SRP). Snapshot deltas are the original point-in-time version of data that has been updated. Multiple snapshots can share a single snapshot delta.
SnapVX uses a combination of redirect-on-write (ROW) and asynchronous copy-on-write (ACOW) to preserve snapshot deltas. When a source write arrives and the original data needs to be preserved as a snapshot delta for multiple snapshots, only a single snapshot delta needs to be created.
ROW: When application data is updated, the new write is accepted and asynchronously written to a new location in the SRP. Application volumes point to the new data, and the original data becomes a snapshot delta while remaining in its original location.
ACOW: When application data is updated, the new write is accepted, and the original data is asynchronously copied to a new location in the SRP as a snapshot delta. The new data is written to the original location.
ROW benefits performance by reducing back-end operations and cache usage. ACOW also benefits performance in some situations, for example, preventing source data from being redirected to a lower performance or compression tier. As new data is written, PowerMaxOS uses the best mechanism for performance and efficient capacity.
When snapshots are terminated, the snapshot delta cleanup process is a lower priority task designed to give priority to other operations in the array, such as host I/O, for example. Snapshot termination does not affect performance regardless of how many snaps are being terminated. The rate at which snapshot capacity is freed depends available system resources. The rate may vary across systems, and on an individual system during different periods of activity.
Factors used to identify capacity needed to support the protection requirements of the applications include:
A snapshot could potentially use capacity equal to the source volume capacity. However, the efficiency features in PowerMaxOS greatly reduce snapshot data in the system.
The SnapVX outputs report the deltas for each snapshot. The “Non-Shared Tracks” value indicates capacity unique to each snapshot, which can be used to determine which snapshots have the most nonshared space to free the most capacity when terminated.
In the following example of the Storage Group Data Protection page in Unisphere, the delta value is based on 128 KB track size. Data that is shared across multiple snapshots is reported under each snapshot. These values should not be used to determine back-end snapshot usage which is explained in the next section.
Note: Capacity values are reported in engineering values (TiB).
Other volumes in the array may also be accessing nonshared snapshot deltas. For example, clones, linked targets, or other volumes through deduplication. This PowerMaxOS functionality greatly improves back-end efficiency. Snapshot deltas shared with other volumes are not released when the snapshot is terminated.
Conditions that prevent snapshot data from being freed to SRP available capacity include:
The reported used snapshot capacity may be less than the sum of the deltas per snapshot due to efficiency of features such as shared allocations and Data Reduction.
Hover over the Snapshot bar graph on the Capacity Dashboard in Unisphere for high-level details. The snapshot values are defined as:
Select the bar graphs to view the Effective Capacity dashboard and Snapshot Capacity dashboard.
On the Effective Capacity dashboard:
The values in the Snapshot Capacity page are defined as: