Portable Media Storage User Manual

RAIDBank5 Owner’s Manual

Appendix C- Glossary

ATA Acronym for “AT Bus Attachment” - a standard interface to IDE hard disks.

Cache cache is a fast-access memory bank that serves as an intermediate storage for data that

is read from or written to secondary storage. Typically, high-speed caches are implemented in

RAM, though they can also be implemented on disk when speed is not a critical requirement.

Caches generally improve the efficiency of read operations due to the principles of “spatial

and temporal locality of data”. They can also improve the efficiency of write operations. See

also: Write Back Cache, Write Through Cache

Degraded Mode/Status All arrays, with the exception of RAID 0, are designed to handle

disk failures. However, there is limit on the number of hard disks that can fail before the

array is rendered inoperative. For instance, this limit value is 1 for RAID 1, 3, and 5. In the

case of RAID 10 or 50, the upper bound is equal to the number of parity groups. When the

number of disk failures occurring in an array are less than or equal to this upper bound, the

array is denoted to be in a degraded state. The failure of the disks does not impair reading

from or writing to the array. However, it impairs the efficiency of throughput in all RAID

types (with the exception of RAID 1) since data requested by read operations may have to be

“reconstructed” using parity. In the case of RAID 1 the throughput of read operations is cut in

half if a drive fails. Operating in degraded mode is considered an acceptable alternative only

for short durations. Generally this duration should span no more time than that required to

inform the user of the failures and to replace the failed disks with suitable spares.

Disk Array A Disk Array is a logical disk comprised of multiple physical hard disks. The

number of hard disks in an disk array is dictated by the type of the array and the number of

spares that may be assigned to it. Furthermore, whether a disk array can be built using part

of the space on a disk (as opposed to being forced to use the whole disk) depends upon the

implementation. Disk Arrays are typically used to provide data redundancy and/or enhanced

I/O performance.

Disk Block Data is stored on disks in blocks that are generally of a predefined size. This size

is typically a value such as 512 bytes, 1 KB, 2 KB, etc. When a record is written to a disk, the

blocks used for that record are dedicated to storing the data for that record only. In other

words two records are not permitted to share a block. Consequently, a block may be only

partially used. For instance, assume a disk has a block size of 1 KB and a user record written

to it has a size of 3148 bytes. This implies that the user record will be written into 4 blocks,

with the contents of one of the blocks being only partially filled with (3148 – 3072) 76 bytes

of data.

Driver A piece of software that controls a hardware device. Typically drivers provide an interface

by which applications can use the device in a uniform and hardware-independent manner.

C-Glossary