Stable storage
Encyclopedia
Stable storage is a classification of computer data storage technology that guarantees atomicity for any given write operation and allows software to be written that is robust
against some hardware and power failures. To be considered atomic, upon reading back a just written-to portion of the disk, the storage subsystem must return either the write data or the data that was on that portion of the disk before the write operation.
Most computer disk drives are not considered stable storage because they do not guarantee atomic write; an error could be returned upon subsequent read of the disk where it was just written to in lieu of either the new or prior data.
.
Most often though, stable storage functionality is achieved by mirroring
data on separate disks via RAID
technology (level 1 or greater). The RAID controller implements the disk writing algorithms that enable separate disks to act as stable storage.
The RAID technique is robust against some single disk failure
in an array of disks whereas the software technique of writing to separate areas of the same disk only protects against some kinds of internal disk media failures such as bad sectors
in single disk arrangements.
Robustness (computer science)
In computer science, robustness is the ability of a computer system to cope with errors during execution or the ability of an algorithm to continue to operate despite abnormalities in input, calculations, etc. Formal techniques, such as fuzz testing, are essential to showing robustness since this...
against some hardware and power failures. To be considered atomic, upon reading back a just written-to portion of the disk, the storage subsystem must return either the write data or the data that was on that portion of the disk before the write operation.
Most computer disk drives are not considered stable storage because they do not guarantee atomic write; an error could be returned upon subsequent read of the disk where it was just written to in lieu of either the new or prior data.
Implementation
Multiple techniques have been developed to achieve the atomic property from weakly atomic devices such as disks. Writing data to a disk in two places in a specific way is one technique and can be done by application softwareApplication software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
.
Most often though, stable storage functionality is achieved by mirroring
Disk mirroring
In data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical hard disks in real time to ensure continuous availability...
data on separate disks via RAID
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
technology (level 1 or greater). The RAID controller implements the disk writing algorithms that enable separate disks to act as stable storage.
The RAID technique is robust against some single disk failure
Hard disk failure
In computing, a hard-disk failure occurs when a hard disk drive malfunctions and the stored information cannot be accessed with a properly configured computer...
in an array of disks whereas the software technique of writing to separate areas of the same disk only protects against some kinds of internal disk media failures such as bad sectors
Disk sector
In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector...
in single disk arrangements.