Managed disk (MDisk) groups Storage units Virtual disks (VDisks)

An MDisk group is a collection of MDisks that jointly contain all the data for a specified set of virtual disks (VDisks). All MDisks in a group are split into extents of the same size. VDisks are created from the extents that are available in the group. You can add MDisks to an MDisk group at any time. This way you increase the number of extents that are available for new VDisks or to expand existing VDisks.

Note:Note: RAID array partitions on HP StorageWorks subsystems controllers are only supported in single-port attach mode. MDisk groups that consist of single-port attached subsystems and other storage subsystems are not supported.

You can add MDisks to an MDisk group at any time either to increase the number of extents that are available for new VDisks or to expand existing VDisks. You can add only MDisks that are in unmanaged mode. When MDisks are added to a group, their mode changes from unmanaged to managed.

You can delete MDisks from a group under the following conditions:

Attention: Attention: If you delete an MDisk group, you destroy all the VDisks that are made from the extents that are in the group. If the group is deleted, you cannot recover the mapping that existed between extents that are in the group and the extents that VDisks use. The MDisks that were in the group are returned to unmanaged mode and can be added to other groups. Because the deletion of a group can cause a loss of data, you must force the deletion if VDisks are associated with it.

The status of an MDisk group consists of three settings. The following table describes the different states of an MDisk group:

Table 2. Managed disk group status
Status Description
Online The MDisk group is online and available. All the MDisks in the group are available.
Degraded The MDisk group is available; however, one or more nodes cannot access all the MDisks in the group.
Offline The MDisk group is offline and unavailable. No nodes in the cluster can access the MDisks. The most likely cause is that one or more MDisks are offline or excluded.
Attention: Attention: If a single MDisk in an MDisk group is offline, that is, it cannot be seen by all of the online nodes in the cluster, the MDisk group that this MDisk is a member of goes offline. This causes all the VDisks that are being presented by this MDisk group to go offline. Care should be taken when creating MDisk groups to ensure an optimal configuration.
Extent

To track the space that is available, the SAN Volume Controller divides each MDisk in an MDisk group into chunks of equal size. These chunks are called extents, and are indexed internally. Extent sizes can be 16, 32, 64, 128, 256, or 512 MB.

You must specify the extent size when you create a new MDisk group. You cannot change the extent size later; it must remain constant throughout the lifetime of the MDisk group. MDisk groups can have different extent sizes, however different extent sizes can place restrictions on the use of data migration. The choice of extent size affects the total amount of storage that can be managed by a SAN Volume Controller cluster. Table 3 shows the maximum amount of storage that can be managed by a cluster for each extent size. Because the SAN Volume Controller allocates a whole number of extents to each virtual disk that is created, using a larger extent size can increase the amount of wasted storage at the end of each virtual disk. Larger extent sizes also reduce the ability of the SAN Volume Controller to distribute sequential I/O workloads across many managed disks. Therefore, larger extent sizes might reduce the performance benefits of virtualization.

Table 3. Capacities of the cluster given extent size
Extent size Maximum storage capacity of cluster
16 MB 64 TB
32 MB 128 TB
64 MB 256 TB
128 MB 512 TB
256 MB 1 PB
512 MB 2 PB

The following figure shows an MDisk group containing four MDisks.

Figure 4. MDisk group
MDisk group
Related topics Storage units Virtual disks (VDisks)

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