Administrator Guide

NOTE: The physical capacity limit for a virtual pool is 512 TiB. When overcommit is enabled, the logical capacity limit is 1
PiB.
● 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.
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.
22 Getting started