Administrator Guide

Virtual pools and disk groups
The volumes within a virtual pool are allocated virtually (separated into fixed size pages, with each page allocated randomly from
somewhere in the pool) and thinly (meaning that they initially exist as an entity but don't have any physical storage allocated to them).
They are also allocated on-demand (as data is written to a page, it is allocated).
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 9 (base volume plus 8 snapshots). The maximum number of volumes per snapshot will
decrease to fewer than 9 if more than 3 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 EMC PowerVault ME4 Series Storage System
CLI Guide.
NOTE: The physical capacity limit for a virtual pool is 512 TiB. When overcommit is enabled, the logical capacity limit is 1
PiB.
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.
Linear pools and disk groups
Each time that the system adds a linear disk group, it also creates a corresponding pool for the disk group. Once a linear disk group and
pool exists, volumes can be added to the pool. The volumes within a linear pool are allocated in a linear way, such that the disk blocks are
sequentially stored on the disk group.
Linear storage maps logical host requests directly to physical storage. In some cases the mapping is one-to-one, while in most cases the
mapping is across groups of physical storage devices, or slices of them.
About volumes and volume groups
A volume is a logical subdivision of a virtual or linear pool and can be mapped to host-based applications. A mapped volume provides
addressable storage to a host (for example, a file system partition you create with your operating system or third-party tools). For more
information about mapping, see About volume mapping.
Virtual volumes
Virtual volumes make use of a method of storing user data in virtualized pages. These pages may be spread throughout the underlying
physical storage in a random fashion and allocated on demand. Virtualized storage therefore has a dynamic mapping between logical and
physical blocks.
Because volumes and snapshots share the same underlying structure, it is possible to create snapshots of other snapshots, not just of
volumes, creating a snapshot tree.
A maximum of 1024 virtual volumes can exist per system.
Volume groups
You can group a maximum of 1024 volumes (standard volumes, snapshots, or both) into a volume group. Doing so enables you to perform
mapping operations for all volumes in a group at once, instead of for each volume individually.
A volume can be a member of only one group. All volumes in a group must be in the same virtual pool. A volume group cannot have the
same name as another volume group, but can have the same name as any volume. A maximum of 256 volume groups can exist per
system. If a volume group is being replicated, the maximum number of volumes that can exist in the group is 16.
NOTE: Volume groups apply only to virtual volumes. You cannot add linear volumes to a volume group.
Getting started 21