eG Monitoring
 

Measures reported by DiskMirrorTest

Disk mirroring is a technique that uses data redundancy (two complete copies of all data stored on two separate disks) to protect against loss of data due to disk failure. One logical volume is duplicated on two separate disks (primary and secondary configuration). Whenever the operating system needs to write to a mirrored volume, both disks are updated. The disks are maintained at all times with exactly the same information. When the operating system needs to read from the mirrored volume, the operating system reads from whichever disk is more readily accessible at the moment, which can result in enhanced performance for read operations.

If a single disk fails, the data is still available, but if both disks fail, then data loss is imminent. To prevent such an eventuality, administrators need to keep track of the status of every disk mirror, rapidly identify failed/unavailable disks, and ensure that they are replaced quickly. This can be achieved using the DiskMirrorTest. This test monitors and reports the current state of each disk mirror, and thus points to failed/unavailable disks and those that have errors. In the process, the test warns administrators of any data loss that may occur owing to the unavailability of disks or disk errors.

This test is disabled by default. To enable the test, select the Enable / Disable option from the Tests menu of the Agents tile in the Admin tile menu. Select Solaris as the Component type, and pick Performance as the Test type. From the list of DISABLED TESTS, pick this test and click the < button to enable it. Finally, click Update.

The measures made by this test are as follows:

Measurement Description Measurement Unit Interpretation
State Indicates the current state of this disk mirror.   The values that this measure can take and their corresponding numeric values are as follows:

Measure Value Numeric Value
Okay 0
Needs Maintenance 1
Last erred 2
Unavailable 3

If a disk fails, it typically switches to the Unavailable state. When a disk experiences errors, Solaris puts that disk in the Maintenance state. No further reads or writes are performed to a disk in the Maintenance state. Sometimes a disk goes into a Last Erred state. For a RAID-1 volume, this usually occurs with a one-sided mirror. The disk experiences errors; however, there are no redundant components to read from. For a RAID-5 volume, this occurs after one disk goes into Maintenance state, and another disk fails. The second disk to fail goes into the Last Erred state.

Note:

By default, this measure reports the Measure Values listed in the table above to indicate the current state of the disk mirror. In the graph of this measure however, the same is represented using the corresponding numeric equivalents only.