Lenovo ThinkServer RD650, User Manual

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LSI Corporation
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12Gb/s MegaRAID SAS Software User Guide
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March 2014
Glossary
C
Cache Fast memory that holds recently accessed data. Use of cache memory speeds subsequent
access to the same data. When data is read from or written to main memory, a copy is also
saved in cache memory with the associated main memory address. The cache memory
software monitors the addresses of subsequent reads to see if the required data is
already stored in cache memory. If it is already in cache memory (a cache hit), it is read
from cache memory immediately and the main memory read is aborted (or not started).
If the data is not cached (a cache miss), it is fetched from main memory and saved in
cache memory.
Cache flush interval A controller property that indicates how often the data cache is flushed.
Caching The process of using a high speed memory buffer to speed up a computer system’s
overall read/write performance. The cache can be accessed at a higher speed than a drive
subsystem. To improve read performance, the cache usually contains the most recently
accessed data, as well as data from adjacent drive sectors. To improve write performance,
the cache can temporarily store data in accordance with its write back policies.
Capacity A property that indicates the amount of storage space on a drive or virtual drive.
Coerced capacity A drive property indicating the capacity to which a drive has been coerced (forced) to
make it compatible with other drives that are nominally the same capacity. For example,
a 4-GB drive from one manufacturer might be 4,196 MB, and a 4-GB from another
manufacturer might be 4,128 MB. These drives could be coerced to a usable capacity of
4,088 MB each for use in a drive group in a storage configuration.
Coercion mode A controller property indicating the capacity to which drives of nominally identical
capacity are coerced (forced) to make them usable in a storage configuration.
Consistency check An operation that verifies that all stripes in a virtual drive with a redundant RAID level are
consistent and that automatically fixes any errors. For RAID 1 drive groups, this operation
verifies correct mirrored data for each stripe.
Consistency check
rate The rate at which consistency check operations are run on a computer system.
Controller A chip that controls the transfer of data between the microprocessor and memory or
between the microprocessor and a peripheral device such as a drive. RAID controllers
perform RAID functions such as striping and mirroring to provide data protection.
Copyback The procedure used to copy data from a source drive of a virtual drive to a destination
drive that is not a part of the virtual drive. The copyback operation is often used to create
or restore a specific physical configuration for a drive group (for example, a specific
arrangement of drive group members on the device I/O buses). The copyback operation
can be run automatically or manually.
Typically, a drive fails or is expected to fail, and the data is rebuilt on a hot spare. The
failed drive is replaced with a new drive. 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.
The copyback operation runs as a background activity, and the virtual drive is still
available online to the host.
Current Measure of the current flowing to (+) or from (-) the battery, reported in milliamperes.
Current write policy A virtual drive property that indicates whether the virtual drive currently supports Write
Back mode or Write Through mode.
 In Write Back mode, the controller sends a data transfer completion signal to the host when
the controller cache has received all of the data in a transaction.

In Write Through mode, the controller sends a data transfer completion signal to the host
when the drive subsystem has received all of the data in a transaction.