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RAID 6 stripes at the block level and then stores two blocks of parity, one more than for RAID5. Parity is interspersed within the data, so for each run of data blocks there are two blocks of parity data, but for each stripe of the RAID 6 Array these are located on different drives. This method is known as distributed parity.

RAID 6 is basically enhanced RAID 5. The additional error correction information means that two disks can fail and the RAID will still operate. If one disk is lost then the RAID works as a RAID 5, if two are lost then it is a degraded RAID 5.

RAID 6 data recovery

RAID 6 does not appear to be widely used, so data recovery requirements are not frequent. From the view of RAID data recovery, RAID 6 increases the likelihood of success over RAID5 with the additional of the extra parity block. Whilst RAID systems tend to be rather cut-and-dried about disks with errors, often they will just stop using the disks so if you have 3 disks developing problems the whole RAID could become inaccessible. The data recovery process can be relatively sophisticated making decisions regarding which sectors to use or whether to re-calculate missing data on a sector by sector basis and often this means that a RAIDS reporting 3 bad disks will be sent in for data recovery and the result will be a 100% success.

Bad Sectors

Damage caused by contact between the hard drive read/write heads and the disk platter, and flaws within the recorded data surface, can result in disk sectors becoming unreadable. This will be treated as a disk failure and the RAID will then run only in degraded mode, so ceasing to access the disk with errors and re-creating the data it holds "on-the-fly" in response to read requests. If three or more disks reports errors then the RAID6 error correction can no longer cope and so the RAID will cease to function. The key point here for RAID data recovery is that only where there are corresponding failed sectors on the drives that have problems will there be loss of data. The RAID data recovery process will involve the securing of all available data from each of the drives and then a rebuilding process that treats all disks as being still active, but that can make decisions on a sector-by-sector basis as to whether the data from all of the drives can be used or, if not, which data to use as part of the rebuilding process.

If there are no corresponding bad sectors then the physical data recovery process will be 100% and so if there are no problems with the file system then the data recovery work is almost complete. In many instances this form of data recovery technique can be used to rebuild the data on new disks that can then be used as replacements within the RAID unit.

Reconfiguration and Rebuilding

RAID rebuilding is a process designed to make a RAID work, but has no consideration for the data. If a RAID is offline and will not start it may be reported that a rebuilding is required. This could be the result of the loss of RAID configuration information from the controller. Rebuilding is not a defined process, the precise manner is down to the RAID hardware that is being used. It may be that the disks are read and the parity information recalculated and written back, it could be that the disks are wiped as part of the process. If the disks are wiped then all data is lost, if the configuration has changed prior to the rebuilding then there will be significant corruption to the data as the pattern of parity distribution across the disks will have changed. The important factor to note is how long the process took, if quick then there will not have been time for damage to a significant proportion of the data, if there was a process that lasted for several hours then there might have been. The RAID data recovery process entail the securing of all data from the disks and then an examination to determine what has happened and how it has affected the data. A full, or significant, recovery of data might be practicable, but it is worth noting that discussing the problem with a RAID data recovery expert prior to embarking upon a RAID rebuild is advisable, there is no obligation to buy a service, and any advise could prove well worthwhile.

Failed Hard Drives

When there are three or more hard disk failures reported a data recovery operation is the only approach that might give access back to the data. RAID6 will function in degraded mode if one or two disks have failed, beyond this then the RAID will fail. One important factor is whether the all failed drives stopped working at the same time or whether any had failed some time previously and the unit had been running for a while with missing drives. Thereafter the data recovery process must concentrate on the hardware and whether any or all of the failed drives have actually stopped working, developed a number of errors, or are actually fine and there is a controller problem.

It is surprising how often during a RAID data recovery diagnosis that we find problems with only one drive though three are reported as failed, quite often we find there are not problems with any of the drives. It appears that some systems respond to electronic problems or instabilities on the "bus" to which the drives are attached by taking drives off-line and asserting error conditions.

If you have a problem with any RAID system it is essential that you seek professional advice. With RAID hardware you do not get much control over what is happening and any steps taken in an attempt to rectify a problem could be a one-way trip to data loss. There is often no way back from an erroneous rebuild.

Last Updated (Thursday, 18 June 2009 15:42)

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