288 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
6.1 Introduction to volumes
A volume is a logical disk that the system presents to attached hosts. For an IBM Spectrum
Virtualize system, the volume presented is from a virtual disk (VDisk). A volume is a discrete
area of usable storage that has been virtualized, using IBM Spectrum Virtualize code, from
storage area network (SAN) storage that is managed by the IBM Spectrum Virtualize cluster.
The term
virtual is used because the volume presented does not necessarily exist on a single
physical entity.
Volumes have the following characteristics or attributes:
Volumes can be created and deleted.
Volumes can be resized (expanded or shrunk).
Volume extents can be migrated at run time to another MDisk or storage pool.
Volumes can be created as fully allocated or thin-provisioned. A conversion from a fully
allocated to a thin-provisioned volume and vice versa can be done at run time.
Volumes can be stored in multiple storage pools (mirrored) to make them resistant to disk
subsystem failures or to improve the read performance.
Volumes can be mirrored synchronously or asynchronously for longer distances. An IBM
Spectrum Virtualize system can run active volume mirrors to a maximum of three other
IBM Spectrum Virtualize systems, but not from the same volume.
Volumes can be copied by using FlashCopy. Multiple snapshots and quick restore from
snapshots (reverse FlashCopy) are supported.
Volumes can be compressed.
Volumes can be virtual. The system supports VMware vSphere Virtual Volumes,
sometimes referred to as VVols, which allow VMware vCenter to manage system objects,
such as volumes and pools. The system administrator can create these objects and
assign ownership to VMware administrators to simplify management of these objects.
Volumes have two major modes: Managed mode and image mode. Managed mode volumes
have two policies: The sequential policy and the striped policy. Policies define how the extents
of a volume are allocated from a storage pool.
Note: A managed disk (MDisk) is a logical unit of physical storage. MDisks are either
Redundant Arrays of Independent Disks (RAID) from internal storage, or external physical
disks that are presented as a single logical disk on the SAN. Each MDisk is divided into
several extents, which are numbered, from 0, sequentially from the start to the end of the
MDisk. The extent size is a property of the storage pools that the MDisks are added to.
Attention: MDisks are not visible to host systems.
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