Glossary

A
access policy		A virtual drive property that indicates what type of access is permitted for a particular virtual drive. The possible values are Read/Write, Read Only, or Blocked.
array		See drive group.
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 whether the required data is already stored in cache memory. If the data 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.
caching		Using a high-speed memory buffer to increase 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 might 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.
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.
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. The MegaRAID Storage Manager software runs on Avago SAS controllers.
D
default write policy		A virtual drive property that indicates whether the default write policy is Write Through or Write Back. 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.
device ID		A controller or drive property that indicates the manufacturer-assigned device ID.
drive group		A group of drives that are attached to a RAID controller on which one or more virtual drives can be created. All virtual drives in the drive group use all of the drives in the drive group.
drive state		A drive property that indicates the status of the drive. A drive can be in one of the following states: Unconfigured Good A drive is accessible to the RAID controller but is not configured as a part of a virtual drive or as a hot spare. Hot Spare A drive that is configured as a hot spare. Online A drive the RAID controller can access and will be part of the virtual drive. Rebuild A drive to which data is being written to restore full redundancy for a virtual drive. Failed A drive that was originally configured as Online or Hot Spare, but on which the firmware detects an unrecoverable error. Unconfigured Bad A drive on which the firmware detects an unrecoverable error; the drive was Unconfigured Good or the drive could not be initialized. Missing A drive that was Online, but which has been removed from its location. Offline A drive that is part of a virtual drive but which has invalid data as far as the RAID configuration is concerned. None A drive with an unsupported flag set. An Unconfigured Good or Offline drive that has completed the prepare for removal operation.
drive type		A drive property that indicates the characteristics of the drive.
device port count		A controller property indicating the number of ports on the controller.
F
Fast initialization		A mode of initialization that quickly writes 0s to the first and last sectors of the virtual drive. This mode lets you to immediately start writing data to the virtual drive while the initialization is running in the background.
fault tolerance		The capability of the drive subsystem to undergo a single drive failure per drive group without compromising data integrity and processing capability. Avago SAS RAID controllers provides fault tolerance through redundant drive groups in RAID levels 1 and 1E. They also support hot spare drives and the auto-rebuild feature.
firmware		Software stored in read-only memory (ROM) or programmable ROM (PROM). Firmware is often responsible for the behavior of a system when it is first turned on. A typical example is a monitor program in a system that loads the full operating system from drive or from a network and then passes control to the operating system.
foreign configuration		A RAID configuration that already exists on a replacement set of drives that you install in a computer system. The MegaRAID Storage Manager software allows you to import the existing configuration to the RAID controller, or you can clear the configuration so you can create a new one.
formatting		The process of writing a specific value to all data fields on a drive, to map out unreadable or bad sectors. Because most drives are formatted when manufactured, formatting is usually done only if a drive generates many media errors.
H
host system		Any computer system on which the controller is installed. Mainframes, workstations, and stand-alone desktop systems all can be considered host systems.
hot spare		A standby drive that can automatically replace a failed drive in a virtual drive and prevent data from being lost. A hot spare can be dedicated to a single redundant drive group or it can be part of the global hot spare pool for all drive groups controlled by the controller.When a drive fails, the MegaRAID Storage Manager software automatically uses a hot spare to replace it and then rebuilds the data from the failed drive to the hot spare. Hot spares can be used in RAID 1 and RAID 1E storage configurations.
I
Initialization		The process of writing 0s to the data fields of a virtual drive and, in fault-tolerant RAID levels, generating the corresponding parity to put the virtual drive in a Ready state. Initialization erases all previous data on the drives. Drive groups will work without initializing, but they can fail a consistency check because the parity fields have not been generated.
IO policy		A virtual drive property that indicates whether Cached I/O or Direct I/O is being used. In Cached I/O mode, all read operations are buffered in cache memory. In Direct I/O mode, read operations are not buffered in cache memory. Data is transferred to cache and the host concurrently. If the same data block is read again, it comes from cache memory. (The IO policy applies to read operations on a specific virtual drive. It does not affect the read-ahead cache.)
M
mirroring		The process of providing complete data redundancy with two drives by maintaining an exact copy of one drive’s data on the second drive. If one drive fails, you can use the contents of the other drive to maintain the integrity of the system and to rebuild the failed drive.
N
NVRAM		Acronym for nonvolatile random access memory. A storage system that does not lose the data stored on it when power is removed. NVRAM stores firmware and configuration data on the RAID controller.
NVRAM size		A controller property that indicates the capacity of the controller’s NVRAM.
O
offline		A drive is offline when it is part of a virtual drive, but its data is not accessible to the virtual drive.
P
patrol read		A process that checks the drives in a storage configuration for drive errors that could lead to drive failure and lost data. The patrol read operation can find and sometimes fix any potential problem with drives before host access. This operation enhances the overall system performance because error recovery during a normal I/O operation may not be necessary.
patrol read rate		The user-defined rate at which patrol read operations are performed on a computer system.
R
RAID		A group of multiple, independent drives that provide high performance by increasing the number of drives used for saving and accessing data. A RAID drive group improves input/output (I/O) performance and data availability. The group of drives appears to the host system as a single storage unit or as multiple virtual drives. Data throughput improves because several drives can be accessed simultaneously. RAID configurations also improve data storage availability and fault tolerance. Redundant RAID levels (RAID levels 1 and 1E) provide data protection.
RAID 0		A RAID 0 drive group uses data striping on one or more drives to provide high data throughput, especially for large files in an environment that requires no data redundancy.
RAID 1		A RAID 1 drive group uses data mirroring on pairs of drives so that data written to one drive is simultaneously written to the other drive. A RAID 1 drive group works well for small databases or other small applications that require complete data redundancy.
RAID 1E		A RAID 1E drive group is the same as a RAID 0 drive group, except that the data is divided into strips, and each strip is then written to one drive and mirrored on another drive. Thus, the data is protected by the mirroring arrangement. As with RAID Level 1, if a drive in the group fails, a hot spare drive can automatically replace it. The RAID 1E drive group performance is roughly the same as for a single drive, although in some instances the dual write operation might be somewhat slower.
RAID 10		A RAID 10 drive group is a spanned drive group that creates a striped set from a series of mirrored drives. The drive group can sustain multiple drive losses as long as no two drives lost comprise a single pair of one mirror.
RAID level		A virtual drive property that indicates the RAID level of the virtual drive. Integrated RAID controllers support RAID levels 0, 1, 1E, and 10.
read policy		A controller attribute that indicates the current Read Policy mode. In Always Read-Ahead mode, the controller reads sequentially ahead of requested data and stores the additional data in cache memory, anticipating that the data will be needed soon. This action speeds up read operations for sequential data, but little improvement exists when accessing random data. In No Read-Ahead mode, the read-ahead capability is disabled. In Adaptive Read-Ahead mode, the controller begins using read-ahead if the two most recent drive accesses occurred in sequential sectors. If the read requests are random, the controller reverts to No Read-Ahead mode.
rebuild		The regeneration of all data to a replacement drive in a redundant virtual drive after a drive failure. A drive rebuild usually occurs without interrupting normal operations on the affected virtual drive, although some degradation of performance of the drive subsystem might occur.
redundancy		The property of a storage configuration that prevents data from being lost when one drive fails in the configuration.
redundant configuration		A virtual drive that has redundant data on drives in the drive group that can be used to rebuild a failed drive. The redundant data can be parity-data-striped across multiple drives in a drive group, or it can be a complete mirrored copy of the data stored on a second drive. A redundant configuration protects the data in case a drive fails in the configuration.
S
SAS		Acronym for Serial Attached SCSI. SAS is a serial, point-to-point, enterprise-level device interface that leverages the Small Computer System Interface (SCSI) protocol set. The SAS interface provides improved performance, simplified cabling, smaller connectors, lower pin count, and lower power requirements when compared to parallel SCSI.
Stripe Size		A virtual drive property that indicates the length of the interleaved data segments that the RAID controller writes across multiple drives, not including parity drives. For example, consider a stripe that contains 64 KB of drive space and has 16 KB of data residing on each drive in the stripe. In this case, the stripe size is 64 KB and the strip size is 16 KB. The user can select the stripe size.
striping		A technique that writes data across all drives in a virtual drive. Each stripe consists of consecutive virtual drive data addresses that are mapped in fixed-size units to each drive in the virtual drive by using a sequential pattern. For example, if the virtual drive includes five drives, the stripe writes data to drives one through five without repeating any of the drives. The amount of space consumed by a stripe is the same on each drive. Striping by itself does not provide data redundancy. Striping in combination with parity does provide data redundancy.
virtual drive		A storage unit created by a RAID controller from one or more drives. Although a virtual drive can be created from several drives, it is seen by the operating system as a single drive. Depending on the RAID level used, the virtual drive might retain redundant data in case of a drive failure.
W
Write-Back		In Write-Back caching mode, the controller sends a data transfer completion signal to the host when the controller cache has received all of the data in a drive write transaction. Data is written to the drive subsystem in accordance with policies that the controller sets up. These policies include the amount of dirty/clean cache lines, the number of cache lines available, and the elapsed time from the last cache flush.
Write Policy		See default write policy.
Write-Through		In Write-Through caching mode, the controller sends a data transfer completion signal to the host when the drive subsystem has received all of the data and has completed the write transaction to the drive.