MDisks

A managed disk (MDisk) is a logical unit of physical storage. MDisks are either arrays (RAID) from internal storage or volumes from external storage systems. MDisks are not visible to host systems.

An MDisk might consist of multiple physical disks that are presented as a single logical disk to the storage area network (SAN). An MDisk always provides usable blocks of physical storage to the system even if it does not have a one-to-one correspondence with a physical disk.

Each MDisk is divided into a number of extents, which are numbered, from 0, sequentially from the start to the end of the MDisk. The extent size is a property of pools. When an MDisk is added to a pool, the size of the extents that the MDisk is divided into depends on the attribute of the pool to which it has been added.

Access modes

The access mode determines how the clustered system uses the MDisk. The following list describes the types of possible access modes:

Unmanaged: The MDisk is not used by the system.
Managed: The MDisk is assigned to a storage pool and provides extents that volumes can use.
Image: The MDisk is assigned directly to a volume with a one-to-one mapping of extents between the MDisk and the volume.
Array: The MDisk represents a set of drives in a RAID from internal storage.

Attention: If you add an MDisk that contains existing data to a storage pool while the MDisk is in unmanaged or managed mode, you lose the data that it contains. The image mode is the only mode that preserves this data.

Figure 1 shows physical disks and MDisks.

Figure 1. Storage Systems and MDisks
This figure shows how MDisks are composed from physical disks

Table 1 describes the operational states of an MDisk.

Table 1. MDisk status
Status	Description
Online	The MDisk can be accessed by all online nodes. That is, all the nodes that are currently working members of the system can access this MDisk. The MDisk is online when the following conditions are met: All timeout error-recovery procedures complete and report the disk as online. Logical unit number (LUN) inventory of the target ports correctly reported the MDisk. Discovery of this LUN completed successfully. All of the MDisk target ports report this LUN as available with no fault conditions.
Degraded paths	The MDisk is not accessible to one or more nodes in the system. Degraded path status is most likely the result of incorrect configuration of either the storage system or the Fibre Channel fabric. However, hardware failures in the storage system, Fibre Channel fabric, or node could also be a contributing factor to this state. To recover from this state, follow these steps: Verify that the fabric configuration rules for storage systems are correct. Ensure that you have configured the storage system properly. Correct any errors in the event log.
Degraded ports	The MDisk has one or more 1220 errors in the event log. The 1220 error indicates that the remote Fibre Channel port has been excluded from the MDisk. This error might cause reduced performance on the storage system and usually indicates a hardware problem with the storage system. To fix this problem, you must resolve any hardware problems on the storage system and fix the 1220 errors in the event log.
Excluded	The MDisk has been excluded from use by the system after repeated access errors. Run the directed maintenance procedures to determine the problem.
Offline	The MDisk cannot be accessed by any of the online nodes. That is, all of the nodes that are currently working members of the system cannot access this MDisk. This state can be caused by a failure in the SAN, storage system, or one or more physical disks connected to the storage system. The MDisk is reported as offline if all paths to the disk fail.

Attention: If you have observed intermittent breaks in links or if you have been replacing cables or connections in the SAN fabric or LAN configuration, you might have one or more MDisks in degraded status. If an I/O operation is attempted when a link is broken and the I/O operation fails several times, the system partially excludes the MDisk and it changes the status of the MDisk to degraded. You must include the MDisk to resolve the problem.

You can include the MDisk by issuing the following command in the command-line interface (CLI):

includemdisk mdiskname/id

where mdiskname/id is the name or ID of your MDisk.

Extents

Each MDisk is divided into chunks of equal size called extents. Extents are a unit of mapping that provide the logical connection between MDisks and volume copies.

MDisk path

Each MDisk from external storage has an online path count, which is the number of nodes that have access to that MDisk; this represents a summary of the I/O path status between the system nodes and the storage device. The maximum path count is the maximum number of paths that have been detected by the system at any point in the past. If the current path count is not equal to the maximum path count, the MDisk might be degraded. That is, one or more nodes might not see the MDisk on the fabric.