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'''Data scrubbing''' is an [[error correction]] technique that uses a background task to periodically inspect [[main memory]] or [[Computer data storage|storage]] for errors, then correct detected errors using [[Data redundancy|redundant data]] in the form of different [[checksum]]s or copies of data. Data scrubbing reduces the likelihood that single correctable errors will accumulate, leading to reduced risks of uncorrectable errors.

[[Data integrity]] is a high-priority concern in writing, reading, storage, transmission, or processing of the [[computer]] [[data]] in computer [[operating system]]s and in computer storage and data transmission systems. However, only a few of the currently existing and used file systems provide sufficient protection against [[data corruption]].

To address this issue, data scrubbing provides routine checks of all [[inconsistency|inconsistencies]] in data and, in general, prevention of hardware or software failure. This "scrubbing" feature occurs commonly in memory, disk arrays, [[file system]]s, or [[Field-programmable gate array|FPGAs]] as a mechanism of error detection and correction.

==RAID==

With data scrubbing, a RAID controller may periodically read all [[hard disk drive]]s in a RAID array and check for defective blocks before applications might actually access them. This reduces the probability of silent data corruption and data loss due to bit-level errors.

In [[Dell PowerEdge]] RAID environments, a feature called "patrol read" can perform data scrubbing and preventive maintenance.

[[Linux MD RAID]], as a [[software RAID]] implementation, makes data consistency checks available and provides automated repairing of detected data inconsistencies. Such procedures are usually performed by setting up a weekly [[cron]] job. Maintenance is performed by issuing operations ''check'', ''repair'', or ''idle'' to each of the examined MD devices. Statuses of all performed operations, as well as general RAID statuses, are always available.

==File systems==

===Btrfs===

As a [[copy-on-write]] (CoW) [[file system]] for [[Linux]], [[Btrfs]] provides fault isolation, corruption detection and correction, and file-system scrubbing. If the file system detects a checksum mismatch while reading a block, it first tries to obtain (or create) a good copy of this block from another device if its internal mirroring or RAID techniques are in use.

Btrfs can initiate an online check of the entire file system by triggering a file system scrub job that is performed in the background. The scrub job scans the entire file system for integrity and automatically attempts to report and repair any bad blocks it finds along the way.

===ZFS===

The features of ZFS, which is a combined [[file system]] and [[logical volume manager]], include the verification against [[data corruption]] modes, continuous integrity checking, and automatic repair. [[Sun Microsystems]] designed ZFS from the ground up with a focus on data integrity and to protect the data on disks against issues such as disk firmware bugs and [[Ghost write (computing)|ghost writes]].

ZFS provides a repair utility called <code>scrub</code> that examines and repairs silent [[data corruption]] caused by [[data degradation|data rot]] and other problems.

==Memory==

Due to the high integration density of contemporary computer memory [[Integrated circuit|chips]], the individual memory cell structures became small enough to be vulnerable to [[cosmic ray]]s and/or [[alpha particle]] emission. The errors caused by these phenomena are called [[soft error]]s. This can be a problem for [[Dynamic random-access memory|DRAM]]- and [[Static random-access memory|SRAM]]-based memories.

''Memory scrubbing'' does error-detection and correction of bit errors in computer [[RAM]] by using [[ECC memory]], other copies of the data, or other error-detecting codes.

==FPGA==

''Scrubbing'' is a technique used to reprogram an [[Field-programmable gate array|FPGA]]. It can be used periodically to avoid the accumulation of errors without the need to find one in the configuration bitstream, thus simplifying the design.

Numerous approaches can be taken with respect to scrubbing, from simply reprogramming the FPGA to partial reconfiguration. The simplest method of scrubbing is to completely reprogram the FPGA at some periodic rate (typically 1/10 the calculated upset rate). However, the FPGA is not operational during that reprogram time, on the order of micro to milliseconds. For situations that cannot tolerate that type of interruption, partial reconfiguration is available. This technique allows the FPGA to be reprogrammed while still operational.

==See also==
* [[Data corruption]]
* [[Silent data corruption]]
* [[Error detection and correction]]
* [[fsck]] - a tool for checking the consistency of a [[file system]]
* [[CHKDSK]] - similar to fsck, used in [[Microsoft Windows|Windows]] operating systems

==Source==

[http://wikipedia.org/ http://wikipedia.org/]
[[Category:Error-detecting codes]]

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