|
Objectives
of Protected Storage Systems
Data
Redundancy
The first basic, and fundamental step to EDAP functionality is to
provide data protection to an array by adding data redundancy -
normally in the form of parity or a complete copy of the
original. This is accomplished by mapping, or uniformly spreading,
data and redundant information across the array's disks. The redundant
information can be a copy of the data, or check data (parity data)
derived from the original data. Additionally, it must be stored
on one or more different disks than the disks storing the data from
which the check data was derived.
Regeneration
The second step to EDAP functionality is regeneration. Without regeneration,
data redundancy is pointless in a critical environment where constant
access is paramount. The objective is for the provision of data
from a failed disk to be made available to users immediately. To
recreate the lost data, the surviving data and check data are processed
on-line. Regeneration also provides for continuous writing to a
failed disk immediately by carefully controlling the user environment
and substituting either spare or virtual drives in the place of
failed ones. This process is transparent to the user. The check
or parity data created by the write operation is stored on a surviving
disk for the tasks of regeneration and reconstruction.
Reconstruction
The third step to EDAP functionality is reconstruction or rebuilding.
It provides for the data from a failed disk to be automatically
written to its replacement disk on-line and in a manner transparent
to users. Reconstruction is a sequence of regeneration tasks in
which the replacement disk is the recipient of all the data from
the failed disk. The sequence, which can be lengthy, commences with
a disk failure and ends when its replacement is rebuilt. Higher
level systems will allow for the option of automatic
regeneration without user intervention and the ability to set
the priority of the rebuilding process.
RAID
sub-systems offer a solution to data redundancy, data regeneration,
and reconstruction after hardware failure. RAID applies to the actual
concept of offering data protection whilst EDAP specifies how this
can be achieved together with strict criteria on the tolerances
the actual physical equipment must offer. Complete specifications
for the production of approved equipment are available from the
RAID
Advisory Board.
Related
topics:
|
