When a

RAID RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...

array experiences the failure of one or more disks, it can enter degraded mode, a fallback mode that generally allows the continued usage of the array, but either loses the performance boosts of the RAID technique (such as a RAID-1 mirror across two disks when one of them fails; performance will fall back to that of a normal, single drive) or experiences severe performance penalties due to the necessity to reconstruct the damaged data from

error correction In information theory and coding theory with applications in computer science and telecommunications, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communi ...

data. Depending on the severity of the problem, the array may be placed into a ''read-only'' mode, either automatically or by the

system administrator An IT administrator, system administrator, sysadmin, or admin is a person who is responsible for the upkeep, configuration, and reliable operation of computer systems, especially multi-user computers, such as Server (computing), servers. The ...

, until it can be corrected. Such corrections may or may not be possible to do ''on-line'' (as opposed to an "off-line" repair, during which the system is unavailable to users). Many RAID configurations feature spare disks that are already installed and can be automatically added to the array as needed; when this happens, the array may or may not go into degraded mode until the spare is rebuilt, but in any case should not be in degraded mode after the spare is rebuilt. If no spares are available, the array will remain in degraded mode until a spare disk is added, or the array is reconfigured (if that is possible for the configuration in question). A typical case where a RAID enters degraded mode is a simple two-drive mirror after a power failure – it is unlikely the drives are in sync. Every time blocks are written to the storage elements (physical drives, in this case), certain accounting information is updated ''after'' the write. The RAID controller will notice that the storage elements are not in sync, will place the array in degraded mode, and – generally – will start a background resync (rebuild) operation. Simple mirroring solutions will resynchronize the entire array, block by block, across both drives, which can be quite time-consuming; this time can be greatly reduced by the usage of a write intent bitmap.Linux RAID Wiki, "Write-intent bitmap", "https://raid.wiki.kernel.org/index.php/Write-intent_bitmap", published March 21, 2011, accessed April 17, 2018

References

{{RAID RAID