MDisks

A managed disk (MDisk) is a logical disk (typically a RAID or partition thereof) that a storage system has exported to the SAN fabric or LAN configuration to which the nodes in the cluster are attached.

An MDisk might, therefore, consist of multiple physical disks that are presented as a single logical disk to the SAN. An MDisk always provides usable blocks of physical storage to the cluster 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 MDisk groups. When an MDisk is added to an MDisk group, the size of the extents that the MDisk is divided into depends on the attribute of the MDisk group to which it has been added.

Access modes

The access mode determines how the cluster uses the MDisk. The following list provides the three types of possible access modes:

Unmanaged: The MDisk is not used by the cluster.
Managed: The MDisk is assigned to an MDisk group and provides extents that virtual disks (VDisks) can use.
Image: The MDisk is assigned directly to a VDisk with a one-to-one mapping of extents between the MDisk and the VDisk.

Attention: If you add an MDisk that contains existing data to an MDisk group 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. Controllers 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 cluster 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 cluster. Degraded path status is most likely the result of incorrect configuration of either the disk controller or the fibre-channel fabric. However, hardware failures in the disk controller, fibre-channel fabric, or node could also be a contributing factor to this state. Complete the following actions to recover from this state: 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 error log.
Degraded ports	The MDisk has one or more 1220 errors in the error 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 controller and usually indicates a hardware problem with the storage controller. To fix this problem you must resolve any hardware problems on the storage controller and fix the 1220 errors in the error log. To resolve these errors in the log, select Service and Maintenance > Run Maintenance Procedures in the SAN Volume Controller Console. On the Maintenance Procedures panel, select Start Analysis. This action displays a list of unfixed errors that are currently in the error log. For these unfixed errors, select the error name to begin a guided maintenance procedure to resolve them. Errors are listed in descending order with the highest priority error listed first. Resolve highest priority errors first.
Excluded	The MDisk has been excluded from use by the cluster 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 cluster 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.

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 VDisk copies.

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 either selecting Work with Managed Disks > Managed Disk > Include an MDisk in the SAN Volume Controller Console, or by issuing the following command in the command-line interface (CLI):

svctask includemdisk mdiskname/id

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

MDisk path

Each MDisk 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 cluster nodes and the storage device. The maximum path count is the maximum number of paths that have been detected by the cluster 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.