14 Implementing the IBM Storwize V5000
1.5.6 Managed disks
An MDisk refers to the unit of storage that IBM Storwize V5000 virtualizes. This unit can be a
logical volume on an external storage array that is presented to the IBM Storwize V5000 or a
RAID array that consists of internal drives. The IBM Storwize V5000 then can allocate these
MDisks into storage pools.
An MDisk is invisible to a host system on the storage area network because it is internal to the
IBM Storwize V5000 system.
An MDisk features the following modes:
Array mode MDisks are constructed from internal drives by using the RAID functionality.
Array MDisks are always associated with storage pools.
LUNs presented by external storage systems to IBM Storwize V5000 are discovered as
unmanaged MDisks. The MDisk is not a member of any storage pools, which means it is
not being used by the IBM Storwize V5000 storage system.
Managed MDisks are LUNs presented by external storage systems to an IBM Storwize
V5000 that are assigned to a storage pool and provide extents so that volumes can use it.
Any data that might be on these LUNs when they are added is lost.
Image MDisks are LUNs that are presented by external storage systems to an IBM
Storwize V5000 and assigned directly to a volume with a one-to-one mapping of extents
between the MDisk and the volume. For more information, see Chapter 6, “Storage
migration wizard” on page 237.
1.5.7 Quorum disks
A quorum disk is an MDisk that contains a reserved area for use exclusively by the system. In
the IBM Storwize V5000, internal drives can be considered as quorum candidates. The
clustered system uses quorum disks to break a tie when exactly half the nodes in the system
remain after a SAN failure.
The clustered system automatically forms the quorum disk by taking a small amount of space
from an MDisk. It allocates space from up to three different MDisks for redundancy, although
only one quorum disk is active.
To avoid the possibility of losing all of the quorum disks because of a failure of a single
storage system if the environment has multiple storage systems, you should allocate the
quorum disk on different storage systems. It is possible to manage the quorum disks by using
1.5.8 Storage pools
A storage pool is a collection of MDisks (up to 128) that are grouped to provide capacity for
volumes. All MDisks in the pool are split into extents of the same size. Volumes are then
allocated out of the storage pool and are mapped to a host system.