Managed Disks; Quorum Disks - IBM V7000 Introduction And Implementation Manual

RAID 6 arrays stripe data over the member drives with two parity stripes (known as the
P-parity and the Q-parity) on every stripe. The two parity strips are calculated using different
algorithms, which give the array double redundancy. A RAID 6 array can be formed by 5 to 16
drives.
RAID 10 arrays have single redundancy. Although they can tolerate one failure from every
mirrored pair, they cannot tolerate two-disk failures. One member out of every pair can be
rebuilding or missing at the same time. A RAID 10 array can be formed by 2 to 16 drives.

2.6.6 Managed disks

A managed disk (MDisk) refers to the unit of storage that IBM Flex System V7000 Storage
Node virtualizes. This unit could be a logical volume from an external storage array presented
to IBM Flex System V7000 Storage Node or a RAID array created on internal drives, or an
external Storwize V7000 expansion that is managed by IBM Flex System V7000 Storage
Node. IBM Flex System V7000 Storage Node can allocate these MDisks into various storage
pools for different usage or configuration needs.
An MDisk should not be visible to a host system on the storage area network, as it should
only be zoned to IBM Flex System V7000 Storage Node system.
An MDisk has four possible modes:
Array:
Array mode MDisks are constructed from drives using the RAID function. Array MDisks
are always associated with storage pools.
Unmanaged:
Unmanaged MDisks are not being used by the system. This situation might occur when an
MDisk is first imported into the system, for example.
Managed:
Managed MDisks are assigned to a storage pool and provide extents so that volumes can
use it.
Image:
Image MDisks are assigned directly to a volume with a one-to-one mapping of extents
between the MDisk and the volume. This situation is normally used when importing logical
volumes into the clustered system that already have data on them, which ensures that the
data is preserved as it is imported into the clustered system.

2.6.7 Quorum disks

A quorum disk is a managed disk (MDisk) that contains a reserved area for use exclusively by
the system. In IBM Flex System V7000 Storage Node, any managed disks can be considered
as a quorum candidate. The clustered system uses quorum disks to break a tie when exactly
half the control canisters in the system remain active after a SAN failure. IBM Flex System
V7000 Storage Node dynamically assigns which quorum drive will be the active member
(DQ).
Chapter 2. Introduction to IBM Flex System V7000 Storage Node
63