MDisks

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

An MDisk might consist of multiple physical disks that are presented as a single logical disk to network. 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 was added.

The access mode determines how the clustered system uses the MDisk.

Attention: If you observed intermittent breaks in links or if you replaced 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 changes the status of the MDisk to excluded. You must include the MDisk to resolve the problem.

You can include the MDisk by entering the following command in the command-line interface (CLI), where mdisk_name is the name or ID of your MDisk.

includemdisk mdisk_name

Access modes

The following list describes the possible access modes for MDisks:

Unmanaged: The MDisk is from an external storage system and 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 from an external storage system and was 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.The MDisk represents a set of drives from internal storage that are managed together by using RAID.

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 examples of 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 from internal storage.

Table 1. Status of MDisks from internal storage
Status	Description
Online	The MDisk is available for I/O and does not require any user attention. All array member drives are configured and the system has redundant access to those drives. The array might have an active background process to synchronize parity.
Degraded paths	The MDisk is available for I/O but requires some user attention. Either the array has missing members, or at least one member drive itself has degraded status. The system does not have full access to the drive.
Offline	The MDisk is not available for I/O. Either too many array member drives are offline or RAID has some internal problem.

Table 2 describes the operational states of an MDisk from external storage systems.

Table 2. Status of MDisks from external storage
Status	Description
Online	The MDisk can be accessed by all online nodes. 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. For iSCSI connections, both nodes of at least one I/O group in the system can access the MDisk.
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 SAN fabric. However, hardware failures in the storage system, SAN fabric, or node might 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 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 was 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 was 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. None of the nodes that are currently working members of the system can access this MDisk. This state can be caused by a failure in the SAN, storage system, or one or more physical disks that are connected to the storage system. The MDisk is reported as offline if all paths to the disk fail.

Extents

Each MDisk is divided into chunks of equal size called extents. Extents are a unit of mapping that provides 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. The path count 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 were 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.