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In their June paper "A Case for Redundant Arrays of Inexpensive Disks RAID ", presented at the SIGMOD conference, they argued that the top performing mainframe disk drives of the time could be beaten on performance by an array of the inexpensive drives that had been developed for the growing personal computer market.
Although failures would rise in proportion to the number of drives, by configuring for redundancy, the reliability of an array could far exceed that of any large single drive. For example, a fast SSD can be mirrored with a mechanical drive.
For this configuration to provide a significant speed advantage an appropriate controller is needed that uses the fast SSD for all read operations. Adaptec calls this "hybrid RAID".
These are called levels. Originally, there were five RAID levels, but many variations have evolved, notably several nested levels and many non-standard levels mostly proprietary. A broken spanned volume at least preserves the files on the unfailing disks.
Data is written identically to two drives, thereby producing a "mirrored set" of drives. Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first depending on its seek time and rotational latencyimproving performance.
Write throughput is always slower because every drive must be updated, and the slowest drive limits the write performance. The array continues to operate as long as at least one drive is functioning.
All disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive. All disk spindle rotation is synchronized and data is striped such that each sequential byte is on a different drive.
Parity is calculated across corresponding bytes and stored on a dedicated parity drive. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost.
Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore.
The elements of a RAID may be either individual drives or arrays themselves. Arrays are rarely nested more than one level deep.
If a single drive failure occurs then one of the stripes has failed, at this point it is running effectively as RAID 0 with no redundancy. The array can sustain multiple drive losses so long as no mirror loses all its drives.
Besides a drastic speed increase, this also provides a substantial advantage:One thing writers hate doing but will inevitably have to do (one day or another, at least) is the Dreaded Synopsis. An agent may request it in his/her submission materials, or an editor might want it once your agent has you out on subs.
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