Data Recovery Weekly

There is a known issue with RAID 5 that if one drive in the array fails completely, then there may be a data loss during the rebuild if one of the remaining drives encounters an unrecoverable read error. These errors are relatively rare, but the size of arrays involved had increased to the point where one cannot even read the entire array without encountering a read error. There are some scary calculations available on the Internet (see an example here ), concluding that there is as much as 50% probability of failing the rebuild on the 12TB (6x2TB) RAID 5 if one disk fails. These calculations are based on somewhat naive assumptions, making the problem look worse than it actually is. First of all, the statement that " There is a 50% probability of being not able to rebuild a 12TB RAID 5 " is the same as " If you have a 10TB RAID 0 array, there is a 50% probability of not getting back what you write to it, if you write the data and then read it back immediately. " Tha...

If a disk fails in a fault-tolerant array such as RAID5, you just replace the disk an carry on. However, if there is a controller failure or an operator error, you might end up with a set of disks lacking the array configuration. You can then send the disk set to the data recovery lab, or try to get the data off yourself using ReclaiMe Free RAID Recovery . The most difficult part of RAID recovery is the destriping, the process of converting the striped array to the contiguous set of sectors as it is on a regular hard drive. ReclaiMe Free RAID Recovery does exactly that, giving you a choice of several output options, and at no cost.

The ability of the RAID to handle failures of its hard drives relies on redundancy of the storage. Once redundancy is lost because of the first drive failure, the human intervention is needed to correct the problem and restore the required level of redundancy. Redundancy is to be restored quickly, or otherwise there is no point in having the RAID at all. However, you do not know when to act if you do not monitor the array and disks often enough. Regularly check the SMART status on the drives using the appropriate software. With a software RAID, use SpeedFan or HDDLife. With a hardware RAID, use the vendor-supplied monitoring software. Any unexplained drop in the throughput may indicate a problem with one of the hard drives.

Even if your RAID is supposed to be fault tolerant, backup often. Although the RAID is redundant with respect to hard drive failures, there are still issues that may bring down the entire array, requiring either a restore from a backup or a RAID recovery . The RAID controller itself is a single point of failure. The failures of the hard drives may be correlated if they have a single root cause (like a power supply failure involving overvoltage). Last not least, RAID does not protect you against human error.

The fault tolerance needs to be tested so that you know exactly how the RAID behaves when a hard drive fails; ensure that the RAID is actually capable of surviving a disk failure. When deploying a fault-tolerant array (RAID 1, RAID 5, or RAID 6), test the system with a simulated disk failure. If you have hot swappable drive bays, just pick a random one and pull the hard drive on a live system. If there is no hot swap available, then disconnect one of the disks with the system powered off. Obviously, you better do the testing before the array is loaded with the production data, or you'd have an unplanned RAID recovery incident.

Do not underestimate the software RAID. Software RAID would typically provide the same, of not better reliability than an entry-level server hardware RAID controller. Software RAID is easier to move around from server to server. It does not require an identical replacement controller in case of a controller failure, which is sometimes a hinderance with server hardware. In RAID 0, RAID 1, and RAID 10, the hardware RAID controller does not provide any computational power benefit because there is nothing to offload. Most modern SOHO and small business NAS devices, like Synology, QNAP, or NetGear, use the software RAID (Linux/mdadm).

Hot spares are a good addition to a fault-tolerant array. If a drive has failed in a fault-tolerant (RAID 1 or RAID 5) array, there is a vulnerability window. If another drive fails during this vulnerability window, the data is lost. Hot spare drives allow the controller to rebuild the array without administrator intervention, thereby reducing the vulnerability window. The need for a hot spare increases as the number of disks in array increases. Hot spares are most effective when a single hot spare drive is shared between several arrays. Consider for example an 8-bay NAS. If there is only one RAID 5 array in the NAS, then RAID 6 may be a better option than a hot spare. The hot spare drive just sits there idly. In a RAID 6 array, the same drive would be utilized to improve a read speed. However if you need two RAID 5 arrays, the hot spare drive is shared between these two arrays, reducing the disk space overhead.

If you plan on building RAID 5 with a total capacity of more than 10 TB, consider RAID 6 instead. The problem with RAID 5 is that once the member disk had failed, it is required to read the entire array in order to complete the RAID rebuild. Although the probability of encountering a read error in any particular read operation is very low, the chance of a read error occurring increases as the array size increases. It has been widely speculated that the probability of encountering the read error during rebuild becomes practically significant as the array size approaches 10TB. Although the speculation relies on certain assumption which is not likely to be true (we'll have a writeup on that later), consider being better safe than sorry . RAID 6, being capable of correcting two simultaneous read errors, does not have this problem.

If you are planning to build a RAID 0, consider using an SSD instead. Depending on what your requirements are, you may find a better bang for your buck with just one SSD. Also, higher RPM rotational drives (e.g. WD VelociRaptor series) or hybrid drives (like Seagate Momentus XT) may be interesting.

The relationship between Speed, Price, and Fault Tolerance mostly determines the RAID level to use. Of these three parameters, pick any two. Fast and Fault Tolerant - RAID 1+0 Fast and Cheap - RAID 0 Cheap and Fault Tolerant - RAID 5 or RAID 6.

When you are about to build a RAID, make sure you understand your storage requirements. The following points are significant Array capacity. Whatever your capacity requirements are, these are underestimated, most likely by the factor of two. Budget limits. Expected activity profile, especially read-to-write ratio. If there are mostly reads and few writes, then RAID 5 or RAID 6 would be OK. If significant random write activity is expected, consider RAID 10 instead. Expected lifetime. Whatever the projected lifetime of the storage system is, it is underestimated. For a quick estimation of capacities for various RAID levels, check the online RAID calculator .