Dell PowerVault ME4 Series Storage System Administrator’s Guide

Virtual pools and disk groups

The volumes within a virtual pool are allocated virtually and separated into fixed size pages, with each page allocated randomly from somewhere in the pool. The volumes within a virtual pool are also thin provisioned, which means that the volumes initially exist as an entity, but physical storage is not allocated to them. The thin-provisioned volumes are allocated on-demand as data is written to a page.

If you would like to create a virtual pool that is larger than 512 TiB on each controller, you can enable the large pools feature by using the large-pools parameter of the set advanced-settings CLI command. When the large pools feature is disabled, which is the default, the maximum size for a virtual pool is 512 TiB, and the maximum number of volumes per snapshot tree is 255 (base volume plus 254 snapshots). Enabling the large pools feature will increase the maximum size for a virtual pool to 1024 TiB (1 PiB) and decrease the maximum number of volumes per snapshot tree to nine, which is the base volume plus eight snapshots. The maximum number of volumes per snapshot decreases to fewer than nine if more than three replication sets are defined for volumes in the snapshot tree. For more information about the large-pools parameter of the set advanced-settings CLI command, see the Dell PowerVault ME4 Series Storage System CLI Guide.

• When the overcommit feature is disabled, the host does not lose read or write access to the pool volumes when the pool reaches or exceeds the high threshold value.
• When the overcommit feature is enabled, the storage system sends the data protect sense key Add, Sense: Space allocation failed write protect to the host when the pool reaches or exceeds the high threshold value. If the host is rebooted after the pool reaches or exceeds the high threshold value, the host loses read and write access to the pool volumes. The only way to regain read and write access to the pool volumes is to add more storage to the pool.

You can remove one or more disk groups, but not all, from a virtual pool without losing data if there is enough space available in the remaining disk groups to contain the data. When the last disk group is removed, the pool ceases to exist, and will be deleted from the system automatically. Alternatively, the entire pool can be deleted, which automatically deletes all volumes and disk groups residing on that pool.

If a system has at least one SSD, each virtual pool can also have a read-cache disk group. Unlike the other disk group types, read-cache disk groups are used internally by the system to improve read performance and do not increase the available capacity of the pool.