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RAID 5 Guide
RAID 5 is a popular multi hard disk drive server configuration designed to militate against potentially catastrophic hardware (single hard drive) failure. A representation of the general storage arrangements is shown in the illustration below. The stored data is striped across the drives in the RAID 5 array as illustrated. In the event of a drive failure the system automatically rebuilds, providing service in what is termed a degraded (not secure) mode. Despite its popularity however here at Datlabs we see a considerable number of such systems that have failed and need our specialist data recovery services.
RAID 5 Disk Array Explained
A RAID 5 data storage system is a multi hard disk drive configuration (Array) of normally 4 x hard drives or above. System data is written to the individual hard disk drives under the control of a specific hardware or software RAID 5 controller. The RAID 5 controller ensures that the system data is written (stored) in blocks sequentially from one drive and then another, this is called striping.
Additional to the primary data each stored data block contains additional propriety parity coding algorithms who’s purpose is to provide data integrity and to facilitate ‘data redundancy.’ The primary system data is broken down into elements called stripes and these elements are distributed throughout the individual hard drive units, the overall data integrity is completed with a separate parity code stripe index. This additional area of parity coding is calculated using the data stored across the disks within the striped volume and is used in the event of a hard disk drive failure to rebuild the data set in full.
RAID 5 Rebuild Failure.
Under correct operation a RAID 5 configured system will be fault tolerant to the failure of at least one unit hard drive in its Array. Resulting delays in the auto rebuild process or other common events such as power outages, however, can result in poor maintenance decisions that ultimately leave the RAID Server unable to rebuild.
Recovering such a failed RAID 5 involves reconstructing the original data striping and the associated parity fields. Here at Datlabs we have a specialist RAID recovery team on hand to help with all aspects of a RAID 5 failure, rebuild and recovery that can reunite you with your lost data.
RAID 5 RECOVERY AND REBUILD
Under correct operation a RAID 5 system is tolerant to the failure of at least one unit hard disk drive in its array. Multiple drive failures and other unscheduled events can cause operating or file system corruption that can preclude a successful rebuild.
Rebuilding and Recovering the data from a failed RAID 5 system involves reconstructing the original operating conditions and repairing data striping and parity field descriptions. Here at Datlabs we have experienced specialists who specifically undertake this work successfully on a regular basis
RAID 5 Rebuild Explained.
If you are a fair way along dealing with a failure situation a bit more information never goes amiss !
RAID 5 Rebuild Limitations.
A rebuild does not repair the file system or make inaccessible data , accessible.
Any data that is missing prior to a rebuild will not be evident after a rebuild.
A rebuild will not:
Fix corrupt files or partitions.
Make your server boot if it wasn’t booting in the first place
If the array is not mounting, server not bootable or recently updated files are now corrupt or inaccessible, a rebuild WILL render the failure permanent.
RAID 5 Faulty Hard Disk Drive Replacement.
A RAID-5 rebuild will take a degraded array and restore redundancy. A RAID-5 rebuild will perform XOR calculations on the degraded set and write those values onto the new, healthy drive you just inserted when you replaced a failed one. Unless the array is accessible and all of the important, recently updated data is valid, never run a RAID rebuild.
Testing a RAID 5 Back-up using another Volume
The most common mistake made by Datlabs clients is to replace two failed Hard Disk Drives in a RAID-5 array and restore from backup. Having restored hundreds of GB of data from the backup onto the newly rebuilt array they discover that the backup was corrupted, incomplete, or out of date..
This problem can be is easily avoided by testing a backup on another storage array other than the original failed array prior to a restore.
Avoid rush decisions to restore to available working drives, simply explain to the client your game-plan is to source a new array, test all the backups ,and then deal with the dead array.
RAID 5 Hot Spares.
Many IT professional take a hot-spare to use in a new storage array, fully confident that it never engaged and is blank . Again, verify your backups are current and consistent on another volume completely unrelated to the failed array before utilizing any of the failed arrays drives, including hot-spares.
Dont Guess Parity / Rotation/Stripe/Off-Sets !
If you are not 100% sure then the odds of you guessing correctly are tiny.
Guessing incorrectly can be catastrophic. The O/S may recognize array or file system corruption and start running repairs which will be catastrophic .
The file system indeed is corrupted from the O/S point of view the file system appears corrupt because you have the wrong configuration.
After these repairs are complete, even if you guess the correct configuration the second time around, it will be too late to salvage any of these file definitions that were repaired.
Don’t Force RAID 5 Hard Drives Online !
Until a backup is verified do not force an offline drive online it is offline for a reason, and was almost certainly failing!
If a hard disk drive failed many days or months ago and you jam it back into the array, all data of relevant size will be corrupted.
Say you have a 3 drive array and the stripe size is 64 kb. Now, you force a drive
that failed months ago online . Any file bigger than 192 kb. will be guaranteed to have stripes of its binary run list residing across all three drives.
Any file bigger than 192 kb created or updated subsequent to the initial drive failure is guaranteed to be incomplete and useless. There will be a 30% chance that the actual file definitions of any file updated since the failure would now appear corrupt or missing. Often the O/S will recognize inconsistencies in the file system and run a helpful check-disk subroutine to repair these problems. However these were not corruptions but inconsistencies and the O/S repairs will likely permanently destroy data across all drives.
Never Plug-In Drives Individually .
Our techs frequently encounter situations where all hard drives in a RAID 5 array have been removed and plugged into a USB chassis as an interface to a data recovery software suite. Not only is this a waste of time but generally results in subsequent actions that cause further problems for us during our attempts to rebuild the system
The O/S may automatically attempt to fix what it is now recognizing as corruptions in the partition table / indexes / master file table. There’s a high probability the drive will show up as un-allocated or available space, and some misinformed IT staff will actually initialize the independent drive with a new volume in order to access its data. The drives were not corrupt in the first place, so fixing the corruptions will typically lead to massive data loss.
Running off-the-shelf data recovery software on a single drive of a 3 drive RAID-5 will yield 1/3 of the file definitions invalid. None of the run-list entries will be correct (file definitions only make sense in the context of the full partition), and the only data available will be definitions of where the data was resident ( “ini” files or log files ).
A Brief Summary:
Approach a failure situation with caution and with full knowledge of how a RAID-5 functions under fault conditions. If the RAID configuration utility warns you that you are about to destroy all the data with a particularly action, don’t do it.
You MUST read and understand the manufacturers manual before doing anything.
You MUST only rebuild to a newly added drive if the volume is good .and running degraded.
Do NOT re-use any hard disk drives from a previously failed volume.
ALWAYS verify your backups on a different set of hardware.
RAID Config Tables Explained:
The following information concerning configuration and stripping is readily available from a number of online resources but is regurgitated here to help consolidate the basics.
RAID 5 is a RAID configuration that combines striping with redundancy. The striping portion of RAID 5 is very similar that of RAID 0, but the redundancy portion is quite different from RAID 1.
RAID 5 systems create redundancy by calculating parity blocks and distributing these parity blocks among all disks in the array. A minimum of three disks is required for a RAID 5 system.
The maximum number of disks is limited by the RAID controller. RAID 5 systems are very popular since they have the performance benefits of striping with the added security of redundancy. Even better is that the storage efficiency (the ratio of the RAID system capacity to the total capacity of all individual disks) is much higher than that of RAID 1 (which is 50%).
Before getting into the details of how data is stored in a RAID 5 system, let’s take a look at parity and how exactly redundant storage efficiency can exceed 50%. Calculating parity is nothing more than applying the XOR binary operator to the data stored on the disks. XOR stands for exclusive OR meaning that the output will equal 1 if and only if the two bits being XOR’d are different. The following is a truth table for the XOR function that illustrates this clearly.
The XOR function has a very unique property that lends itself to efficient data redundancy. If XOR is applied twice in a row, it negates itself. So if we have A and XOR it twice with B, we get A as the result:
A XOR B XOR B = A
The following example will demonstrate how to get 80% redundant storage efficiency out of an array of 5 disks. For 80% efficiency, we must store real data on 4 disks and use only 1 disk for redundancy. Let’s store the string RAID on our 4 non-redundant disks.
The RAID Configuration string in binary:
Here is our string in our disk array at 80% efficiency with the redundancy portion (parity) taking just 20%. The data on Disk E is the parity and is calculated by applying XOR to all the other data like this:
E = A XOR B XOR C XOR D
Now say that Disk C fails and we’re left with the data from Disk A, B, D, and E. We can rebuild the data from Disk C by applying XOR to the data on the remaining disks. Since the data on Disk E = A XOR B XOR C XOR D, applying XOR to all remaining data reduces to
(A XOR B XOR C XOR D) XOR A XOR B XOR D
Now, reformat to
(A XOR A) XOR (B XOR B) XOR (D XOR D) XOR C Since XOR is applied twice in a row to the data from A, B, and D, all we are left with at the end is C. This is of course the data from our failed disk representing the letter “I”. The following chart illustrates the calculation: