Lenovo ThinkServer RD650, User Manual

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LSI Corporation
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12Gb/s MegaRAID SAS Software User Guide

March 2014
Chapter 2: Introduction to RAID
Components and Features
In a system with two ESMs, you can replace one of the ESMs without affecting the virtual drive availability. For
example, the controller can run heavy I/Os, and when you replace one of the ESMs, I/Os should not stop. The
controller uses different paths to balance the load on the entire system.
In the MegaRAID Storage Manager utility, when multiple paths are available to a drive, the drive information shows
only one enclosure. The utility shows that a redundant path is available to a drive. All drives with a redundant path
display this information. The firmware supports online replacement of enclosure modules.
2.1.4 Consistency Check
The consistency check operation verifies correctness of the data in virtual drives that use RAID levels 1, 5, 6, 10, 50, and
60. RAID 0 does not provide data redundancy. For example, in a system with parity, checking consistency means
computing the data on one drive and comparing the results to the contents of the parity drive.
NOTE It is recommended that you perform a consistency check at least once
a month.
2.1.5 Replace
Replace lets you copy data from a source drive into a destination drive that is not a part of the virtual drive. Replace
often creates or restores a specific physical configuration for a drive group (for example, a specific arrangement of
drive group members on the device I/O buses). You can run Replace automatically or manually.
Typically, when a drive fails or is expected to fail, the data is rebuilt on a hot spare. The failed drive is replaced with a
new disk. Then the data is copied from the hot spare to the new drive, and the hot spare reverts from a rebuild drive to
its original hot spare status. Replace runs as a background activity, and the virtual drive is still available online to the
host.
Replace is also initiated when the first SMART error occurs on a drive that is part of a virtual drive. The destination drive
is a hot spare that qualifies as a rebuild drive. The drive that has the SMART error is marked as
failed only after the
successful completion of the Replace. This situation avoids putting the drive group in Degraded status.
NOTE During Replace, if the drive group involved in Replace is deleted
because of a virtual drive deletion, the destination drive reverts to an
Unconfigured Good state or Hot Spare state.
NOTE When Replace is enabled, the alarm continues to beep even after a
rebuild is complete; the alarm stops beeping only when Replace is
completed.
Order of Precedence
In the following scenarios, rebuild takes precedence over Replace:

If a Replace is already taking place to a hot spare drive, and any virtual drive on the controller degrades, the
Replace aborts, and a rebuild starts. Rebuild changes the virtual drive to the Optimal state.

The Rebuild takes precedence over the Replace when the conditions exist to start both operations. Consider the
following examples:
—
Hot spare is not configured (or unavailable) in the system.
—
Two drives (both members of virtual drives) exist, with one drive exceeding the SMART error threshold, and
the other failed.
—
If you add a hot spare (assume a global hot spare) during a Replace, the Replace is ended abruptly, and
Rebuild starts on the hot spare.