Usable storage capacity Disk array capacity that is usable for data storage (vs. for mirroring or parity data). For example, under mirroring (RAID 1 and 0/1), usable storage remains a constant fifty percent (half of storage is always used for redundancy). This is in contrast to other RAID levels such as RAID 5, in which usable storage capacity is determined by the formula of "n-1". "n" is the total number of disk drives and "1" is the number of disks worth of capacity used for parity (redundancy) overhead. So, as the number of disks in the array grows, the usable storage capacity percentage increases in relation to parity (redundancy) information.

Warm swap The ability to remove and replace a disk drive while the power is on. All bus activity must be paused (usually done through a utility within the array management software) to maintain data integrity during removal or replacement. Typically used when hot swap is not supported by the server or storage enclosure drive tray.

Write-back cache A performance caching technique in which the completion of a write request is signaled as soon as the data is in cache; Actual writing to the disk occurs at a later time. Since the operating system is "fooled" into thinking that the write has actually been written to disk, there is a risk of losing or corrupting data in cache should an error or power failure occur. Therefore, use of a battery-backed cache is recommended to prevent such data loss/ corruption.

Write-through cache A caching technique in which the completion of a write request is not signaled until data is safely stored on disk. Performance of write-through is essentially the same as in non-cached systems.

XOR or eXclusive OR An algorithm for treating data, resulting in a true or false verdict. Used extensively in RAID 3 and 5 calculations for error checking and correcting. See Ars Technica for a detailed definition.

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