principal technologies for storing data
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nThere are two principal technologies for storing data redundantly in high availability disk systems:
Hardware-driven redundancy, that is, RAID
This comprises the whole range of RAID technology ("Redundant Array of Inexpensive / Independent Disks"):
•Hardware-emulated mirroring on RAID disks (RAID level 1, see diagram below)
•Error-correcting technology (check bit computing) on RAID systems (levels > 1)
An intelligent disk (that is, controller, storage processors, and microcode) acts independently from the host machine CPU to make sure that data can be retrieved from the device even if one of the disk spindles fails.
Software-driven redundancy, that is, LVM
This is the regime of add-on software for the operating system, namely "Logical Volume Manager" software (LVM). CPU cycles are stolen from the host machine to run LVM software, so ensuring that data is written to physically redundant (mirrored) locations on disk(s). Underlying disk technology can be either several physically independent (standalone) disks or an array of disks sharing common hardware resources (disk tower, rack or cabinet, JBODs) but with no intelligent RAID controller installed on the device.
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