RAID HARD DRIVE DATA RECOVERY
RAID Data recovery involves RAID (redundant array of independent disks, originally redundant array of inexpensive disks is a storage technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on what level of redundancy and performance (via parallel communication) is required.
Data Recovery Pro offers RAID Data recovery from failed, crashed, broken, noisy, clicking, corrupt, formatted, deleted, re-loaded, burnt, fire damaged, power, lightning surge damaged, water damaged, virus damaged RAID 0, RAID 1, RAID 5, RAID 5e, RAID 10, RAID 53 ,SAN, NAS volumes, drives and devices - Striped, spanned, striped and JBOD RAID volumes.
RAID HARD DRIVE DATA RECOVERY
Here are some symptoms that often require RAID DATA RECOVERY:
RAID Controller failure - RAID device not ready - Incomplete or Partial RAID Array Rebuilds - Inaccessible boot device - Unable to access drives in RAID array - Device not ready, reading drives in RAID array - Single and Multiple Raid Drive Failure within array - Operating system not found or missing operating system - Improper drive or media replacement - Fire Damage - Water Damage - NTLDR is missing - RAID device not bootable - Deletion - User error.
RAID is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple physical drives. The physical drives are said to be in a RAID array, which is accessed by the operating system as one single drive. The different schemes or architectures are named by the word RAID followed by a number (e.g., RAID 0, RAID 1). Each scheme provides a different balance between two key goals: increase data reliability and increase input/output performance.
Following is a brief textual summary of the most commonly used RAID levels
RAID 0 (block-level striping without parity or mirroring) has no (or zero) redundancy. It provides improved performance and additional storage but no fault tolerance. Hence simple stripe sets are normally referred to as RAID 0. Any drive failure destroys the array, and the likelihood of failure increases with more drives in the array (at a minimum, catastrophic data loss is almost twice as likely compared to single drives without RAID). A single drive failure destroys the entire array because when data is written to a RAID 0 volume, the data is broken into fragments called blocks. The number of blocks is dictated by the stripe size, which is a configuration parameter of the array. The blocks are written to their respective drives simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off each drive in parallel, increasing bandwidth. RAID 0 does not implement error checking, so any error is uncorrectable. More drives in the array means higher bandwidth, but greater risk of data loss.
RAID 1 (mirroring without parity or striping), data is written identically to two drives, thereby producing a "mirrored set"; at least two drives are required to constitute such an array. While more constituent drives may be employed, many implementations deal with a maximum of only two; of course, it might be possible to use such a limited level 1 RAID itself as a constituent of a level 1 RAID, effectively masking the limitation. The array continues to operate as long as at least one drive is functioning. With appropriate operating system support, there can be increased read performance, and only a minimal write performance reduction; implementing RAID 1 with a separate controller for each drive in order to perform simultaneous reads (and writes) is sometimes called multiplexing (or duplexing when there are only two drives).
RAID 2 (bit-level striping with dedicated Hamming-code parity), all disk spindle rotation is synchronized, and data is striped such that each sequential bit is on a different drive. Hamming-codeparity is calculated across corresponding bits and stored on at least one parity drive.
RAID 3 (byte-level striping with dedicated parity), all disk spindle rotation is synchronized, and data is striped so each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.
RAID 4 (block-level striping with dedicated parity) is identical to RAID 5 (see below), but confines all parity data to a single drive. In this setup, files may be distributed between multiple drives. Each drive operates independently, allowing I/O requests to be performed in parallel. However, the use of a dedicated parity drive could create a performance bottleneck; because the parity data must be written to a single, dedicated parity drive for each block of non-parity data, the overall write performance may depend a great deal on the performance of this parity drive.
RAID 5 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate; the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. However, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt. Additionally, there is the potentially disastrous RAID 5 write hole. RAID 5 requires at least three disks.
RAID 6 (block-level striping with double distributed parity) provides fault tolerance of two drive failures; the array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. This becomes increasingly important as large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single-parity RAID levels are as vulnerable to data loss as a RAID 0 array until the failed drive is replaced and its data rebuilt; the larger the drive, the longer the rebuild takes. Double parity gives additional time to rebuild the array without the data being at risk if a single additional drive fails before the rebuild is complete. Like RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt.
NESTED HYBRID RAID
In what was originally termed hybrid RAID, many storage controllers allow RAID levels to be nested. The elements of a RAID may be either individual drives or RAIDs themselves. Nesting more than two deep is unusual.
As there is no basic RAID level numbered larger than 9, nested RAIDs are usually unambiguously described by attaching the numbers indicating the RAID levels, sometimes with a "+" in between. The order of the digits in a nested RAID designation is the order in which the nested array is built: For a RAID 1+0, drives are first combined into multiple level 1 RAIDs that are themselves treated as single drives to be combined into a single RAID 0; the reverse structure is also possible (RAID 0+1).
The final RAID is known as the top array. When the top array is a RAID 0 (such as in RAID 1+0 and RAID 5+0), most vendors omit the "+" (yielding RAID 10 and RAID 50, respectively).
RAID 0+1: striped sets in a mirrored set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
RAID 1+0: (a.k.a. RAID 10) mirrored sets in a striped set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives
RAID 5+3: mirrored striped set with distributed parity (some manufacturers label this as RAID 53).
Whether an array runs as RAID 0+1 or RAID 1+0 in practice is often determined by the evolution of the storage system. A RAID controller might support upgrading a RAID 1 array to a RAID 1+0 array on the fly, but require a lengthy offline rebuild to upgrade from RAID 1 to RAID 0+1. With nested arrays, sometimes the path of least disruption prevails over achieving the preferred configuration.
Many RAID levels employ an error protection scheme called "parity". Most use the simple XOR parity described in this section, but RAID 6 uses two separate parities based respectively on addition and multiplication in a particular Galois Field or Reed-Solomon error correction. XOR parity calculation is a widely used method in information technology to provide fault tolerance in a given set of data.
SOFTWARE BASED RAID
Software RAID implementations are now provided by many operating systems. Software RAID can be implemented as:
- a layer that abstracts multiple devices, thereby providing a single virtual device (e.g. Linux's md).
- a more generic logical volume manager (provided with most server-class operating systems, e.g. Veritas or LVM).
- a component of the file system (e.g. ZFS or Btrfs).
VOLUME MANAGER SUPPORT
Server class operating systems typically provide logical volume management, which allows a system to use logical volumes which can be resized or moved. Often, features like RAID or snapshots are also supported.
- Vinum is a logical volume manager supporting RAID-0, RAID-1, and RAID-5. Vinum is part of the base distribution of the FreeBSD operating system, and versions exist for NetBSD, OpenBSD, andDragonFly BSD.
- Solaris SVM supports RAID 1 for the boot filesystem, and adds RAID 0 and RAID 5 support (and various nested combinations) for data drives.
- Linux LVM supports RAID 0 and RAID 1.
- HP's OpenVMS provides a form of RAID 1 called "Volume shadowing", giving the possibility to mirror data locally and at remote cluster systems.
FILE SYSTEM SUPPORT
Some advanced file systems are designed to organize data across multiple storage devices directly (without needing the help of a third-party logical volume manager).
- ZFS supports equivalents of RAID 0, RAID 1, RAID 5 (RAID Z), RAID 6 (RAID Z2), and a triple parity version RAID Z3, and any nested combination of those like 1+0. ZFS is the native file system on Solaris, and also available on FreeBSD.
- Btrfs supports RAID 0, RAID 1, and RAID 10 (RAID 5 and 6 are under development).
Many operating systems provide basic RAID functionality independently of volume management.
- Apple's Mac OS X Server and Mac OS X support RAID 0, RAID 1, and RAID 1+0.
- FreeBSD supports RAID 0, RAID 1, RAID 3, and RAID 5, and all nestings via GEOM modules and ccd.
- Linux's md supports RAID 0, RAID 1, RAID 4, RAID 5, RAID 6, and all nestings. Certain reshaping/resizing/expanding operations are also supported.
- Microsoft's server operating systems support RAID 0, RAID 1, and RAID 5. Some of the Microsoft desktop operating systems support RAID such as Windows XP Professional which supports RAID level 0 in addition to spanning multiple drives but only if using dynamic disks and volumes. Windows XP can be modified to support RAID 0, 1, and 5.
- NetBSD supports RAID 0, RAID 1, RAID 4, and RAID 5, and all nestings via its software implementation, named RAIDframe.
- OpenBSD aims to support RAID 0, RAID 1, RAID 4, and RAID 5 via its software implementation softraid.
- FlexRAID (for Linux and Windows) is a snapshot RAID implementation.
Software RAID has advantages and disadvantages compared to hardware RAID. The software must run on a host server attached to storage, and the server's processor must dedicate processing time to run the RAID software; the additional processing capacity required for RAID 0 and RAID 1 is low, but parity-based arrays require more complex data processing during write or integrity-checking operations. As the rate of data processing increases with the number of drives in the array, so does the processing requirement. Furthermore, all the buses between the processor and the drive controller must carry the extra data required by RAID, which may cause congestion.
Fortunately, over time, the increase in commodity CPU speed has been consistently greater than the increase in drive throughput; the percentage of host CPU time required to saturate a given number of drives has decreased. For instance, under 100% usage of a single core on a 2.1 GHz Intel "Core2" CPU, the Linux software RAID subsystem (md) as of version 2.6.26 is capable of calculating parity information at 6 GB/s; however, a three-drive RAID 5 array using drives capable of sustaining a write operation at 100 MB/s only requires parity to be calculated at the rate of 200 MB/s, which requires the resources of just over 3% of a single CPU core.
Furthermore, software RAID implementations may employ more sophisticated algorithms than hardware RAID implementations (e.g. drive scheduling and command queueing), and thus, may be capable of better performance.
Another concern with software implementations is the process of booting the associated operating system. For instance, consider a computer being booted from a RAID 1 (mirrored drives); if the first drive in the RAID 1 fails, then a first-stage boot loader might not be sophisticated enough to attempt loading the second-stage boot loader from the second drive as a fallback. In contrast, a RAID 1 hardware controller typically has explicit programming to decide that a drive has malfunctioned and that the next drive should be used. At least the following second-stage boot loaders are capable of loading a kernel from a RAID 1:
- LILO (for Linux).
- Some configurations of the GRUB.
- The boot loader for FreeBSD.
- The boot loader for NetBSD.
For data safety, the write-back cache of an operating system or individual drive might need to be turned off in order to ensure that as much data as possible is actually written to secondary storage before some failure (such as a loss of power); unfortunately, turning off the write-back cache has a performance penalty that can be significant depending on the workload and command queuing support. In contrast, a hardware RAID controller may carry a dedicated battery-powered write-back cache of its own, thereby allowing for efficient operation that is also relatively safe. Fortunately, it is possible to avoid such problems with a software controller by constructing a RAID with safer components; for instance, each drive could have its own battery or capacitor on its own write-back cache, and the drive could implement atomicity in various ways, and the entire RAID or computing system could be powered by a UPS, etc.
Finally, a software RAID controller that is built into an operating system usually uses proprietary data formats and RAID levels, so an associated RAID usually cannot be shared between operating systems as part of a multi boot setup. However, such a RAID may be moved between computers that share the same operating system; in contrast, such mobility is more difficult when using a hardware RAID controller because both computers must provide compatible hardware controllers. Also, if the hardware controller fails, data could become unrecoverable unless a hardware controller of the same type is obtained.
Most software implementations allow a RAID to be created from partitions rather than entire physical drives. For instance, an administrator could divide each drive of an odd number of drives into two partitions, and then mirror partitions across drives and stripe a volume across the mirrored partitions to emulate IBM's RAID 1E configuration. Using partitions in this way also allows for constructing multiple RAIDs in various RAID levels from the same set of drives. For example, one could have a very robust RAID 1 for important files, and a less robust RAID 5 or RAID 0 for less important data, all using the same set of underlying drives. (Some BIOS-based controllers offer similar features, e.g. Intel Matrix RAID.) Using two partitions from the same drive in the same RAID puts data at risk if the drive fails; for instance:
- A RAID 1 across partitions from the same drive makes all the data inaccessible if the single drive fails.
- Consider a RAID 5 composed of 4 drives, 3 of which are 250 GB and one of which is 500 GB; the 500 GB drive is split into 2 partitions, each of which is 250 GB. Then, a failure of the 500 GB drive would remove 2 underlying 'drives' from the array, causing a failure of the entire array.
HARDWARE BASED RAID
Hardware RAID controllers use proprietary data layouts, so it is not usually possible to span controllers from different manufacturers. They do not require processor resources, the BIOS can boot from them, and tighter integration with the device driver may offer better error handling.
On a desktop system, a hardware RAID controller may be an expansion card connected to a bus (e.g., PCI or PCIe), a component integrated into the motherboard; there are controllers for supporting most types of drive technology, such as IDE/ATA, SATA, SCSI, SSA, Fibre Channel, and sometimes even a combination. The controller and drives may be in a stand-alone enclosure, rather than inside a computer, and the enclosure may be directly attached to a computer, or connected via a SAN.
Most hardware implementations provide a read/write cache, which, depending on the I/O workload, improves performance. In most systems, the write cache is non-volatile (i.e. battery-protected), so pending writes are not lost in the event of a power failure.
Hardware implementations provide guaranteed performance, add no computational overhead to the host computer, and can support many operating systems; the controller simply presents the RAID as another logical drive.
FIRMWARE BASED RAID
A RAID implemented at the level of an operating system is not always compatible with the system's boot process, and it is generally impractical for desktop versions of Windows (as described above). However, hardware RAID controllers are expensive and proprietary. To fill this gap, cheap "RAID controllers" were introduced that do not contain a dedicated RAID controller chip, but simply a standard drive controller chip with special firmware and drivers; during early stage bootup, the RAID is implemented by the firmware, and once the operating system has been more completely loaded, then the drivers take over control. Consequently, such controllers may not work when driver support is not available for the host operating system.
Initially, the term "RAID controller" implied that the controller does the processing. However, while a controller without a dedicated RAID chip is often described by a manufacturer as a "RAID controller", it is rarely made clear that the burden of RAID processing is borne by a host computer's central processing unit rather than the RAID controller itself. Thus, this new type is sometimes called "fake" RAID; Adaptec calls it a "HostRAID".
Moreover, a firmware controller can often only support certain types of hard drive to form the RAID that it manages (e.g. SATA for an Intel Matrix RAID, as there is neither SCSI nor PATA support in modern Intel ICH southbridges; however, motherboard makers implement RAID controllers outside of the southbridge on some motherboards).
Both hardware and software RAIDs with redundancy may support the use of a hot spare drive; this is a drive physically installed in the array which is inactive until an active drive fails, when the system automatically replaces the failed drive with the spare, rebuilding the array with the spare drive included. This reduces the mean time to recovery (MTTR), but does not completely eliminate it. As with non-hot-spare systems, subsequent additional failure(s) in the same RAID redundancy group before the array is fully rebuilt can cause data loss. Rebuilding can take several hours, especially on busy systems.
It is sometimes considered that if drives are procured and installed at the same time, several drives are more likely to fail at about the same time than for unrelated drives, so rapid replacement of a failed drive is important. RAID 6 without a spare uses the same number of drives as RAID 5 with a hot spare and protects data against failure of up to two drives, but requires a more advanced RAID controller and may not perform aswell. Further, a hot spare can be shared by multiple RAID sets.
DATA SCRUB PATROL READ
Data scrubbing is periodic reading and checking by the RAID controller of all the blocks in a RAID, including those not otherwise accessed. This allows bad blocks to be detected before they are used.
An alternate name for this is patrol read. This is defined as a check for bad blocks on each storage device in an array, but which also uses the redundancy of the array to recover bad blocks on a single drive and reassign the recovered data to spare blocks elsewhere on the drive.
PROBLEMS WITH RAID
The theory behind the error correction in RAID assumes that failures of drives are independent. Given these assumptions, it is possible to calculate how often they can fail and to arrange the array to make data loss arbitrarily improbable. There is also an assumption that motherboard failures won't damage the hard drive and that hard drive failures occur more often than motherboard failures.
In practice, the drives are often the same age (with similar wear) and subject to the same environment. Since many drive failures are due to mechanical issues (which are more likely on older drives), this violates those assumptions; failures are in fact statistically correlated. In practice, the chances of a second failure before the first has been recovered (causing data loss) is not as unlikely as for random failures. In a study including about 100 thousand drives, the probability of two drives in the same cluster failing within one hour was observed to be four times larger than was predicted by theexponential statistical distribution which characterizes processes in which events occur continuously and independently at a constant average rate. The probability of two failures within the same 10-hour period was twice as large as that which was predicted by an exponential distribution.
A common assumption is that "server-grade" drives fail less frequently than consumer-grade drives. Two independent studies (one by Carnegie Mellon University and the other by Google) have shown that the "grade" of a drive does not relate to the drive's failure rate.
In addition, there is no protection circuitry between the motherboard and hard drive electronics, so a catastrophic failure of the motherboard can cause the harddrive electronics to fail. Therefore, taking elaborate precautions via RAID setups ignores the equal risk of electronics failures elsewhere which can cascade to a hard drive failure. For a robust critical data system, no risk can outweigh another as the consequence of any data loss is unacceptable.
This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features. Database researcher Jim Gray wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization. However, this warning largely went unheeded and fell by the wayside upon the advent of RAID, which many software engineers mistook as solving all data storage integrity and reliability problems. Many software programs update a storage object "in-place"; that is, they write a new version of the object on to the same secondary storage addresses as the old version of the object. While the software may also log some delta information elsewhere, it expects the storage to present "atomic write semantics," meaning that the write of the data either occurred in its entirety or did not occur at all.
However, very few storage systems provide support for atomic writes, and even fewer specify their rate of failure in providing this semantic. Note that during the act of writing an object, a RAID storage device will usually be writing all redundant copies of the object in parallel, although overlapped or staggered writes are more common when a single RAID processor is responsible for multiple drives. Hence an error that occurs during the process of writing may leave the redundant copies in different states, and furthermore may leave the copies in neither the old nor the new state. The little known failure mode is that delta logging relies on the original data being either in the old or the new state so as to enable backing out the logical change, yet few storage systems provide an atomic write semantic for a RAID.
While the battery-backed write cache may partially solve the problem, it is applicable only to a power failure scenario.
Since transactional support is not universally present in hardware RAID, many operating systems include transactional support to protect against data loss during an interrupted write. Novell NetWare, starting with version 3.x, included a transaction tracking system. Microsoft introduced transaction tracking via the journaling feature in NTFS. ext4 has journaling with checksums; ext3 has journaling without checksums but an "append-only" option, or ext3cow (Copy on Write). If the journal itself in a filesystem is corrupted though, this can be problematic. The journaling in NetApp WAFL file system gives atomicity by never updating the data in place, as does ZFS. An alternative method to journaling is soft updates, which are used in some BSD-derived system's implementation of UFS.
This can present as a sector read failure. Some RAID implementations protect against this failure mode by remapping the bad sector, using the redundant data to retrieve a good copy of the data, and rewriting that good data to the newly mapped replacement sector. The UBE (Unrecoverable Bit Error) rate is typically specified at 1 bit in 1015 for enterprise class drives (SCSI, FC, SAS), and 1 bit in 1014 for desktop class drives (IDE/ATA/PATA, SATA). Increasing drive capacities and large RAID 5 redundancy groups have led to an increasing inability to successfully rebuild a RAID group after a drive failure because an unrecoverable sector is found on the remaining drives. Double protection schemes such as RAID 6 are attempting to address this issue, but suffer from a very high write penalty.
WRITE CACHE RELAIBILITY
The drive system can acknowledge the write operation as soon as the data is in the cache, not waiting for the data to be physically written. This typically occurs in old, non-journaled systems such as FAT32, or if the Linux/Unix "writeback" option is chosen without any protections like the "soft updates" option (to promote I/O speed whilst trading-away data reliability). A power outage or system hang such as a BSOD can mean a significant loss of any data queued in such a cache.
Often a battery is protecting the write cache, mostly solving the problem. If a write fails because of power failure, the controller may complete the pending writes as soon as restarted. This solution still has potential failure cases: the battery may have worn out, the power may be off for too long, the drives could be moved to another controller, and the controller itself could fail. Some systems provide the capability of testing the battery periodically, however this leaves the system without a fully charged battery for several hours.
An additional concern about write cache reliability exists, specifically regarding devices equipped with a write-back cache—a caching system which reports the data as written as soon as it is written to cache, as opposed to the non-volatile medium. The safer cache technique is write-through, which reports transactions as written when they are written to the non-volatile medium.
The methods used to store data by various RAID controllers are not necessarily compatible, so that it may not be possible to read a RAID on different hardware, with the exception of RAID 1, which is typically represented as plain identical copies of the original data on each drive. Consequently a non-drive hardware failure may require the use of identical hardware to recover the data, and furthermore an identical configuration has to be reassembled without triggering a rebuild and overwriting the data. Software RAID however, such as implemented in the Linux kernel, alleviates this concern, as the setup is not hardware dependent, but runs on ordinary drive controllers, and allows the reassembly of an array. Additionally, individual drives of a RAID 1 (software and most hardware implementations) can be read like normal drives when removed from the array, so no RAID system is required to retrieve the data. Inexperienced data recovery firms typically have a difficult time recovering data from RAID drives, with the exception of RAID1 drives with conventional data structure.
DATA RECOVERY IN THE EVENT OF FAILED RAID
With larger drive capacities the odds of a drive failure during rebuild are not negligible. In that event, the difficulty of extracting data from a failed array must be considered. Only a RAID 1 (mirror) stores all data on each drive in the array. Although it may depend on the controller, some individual drives in a RAID 1 can be read as a single conventional drive; this means a damaged RAID 1 can often be easily recovered if at least one component drive is in working condition. If the damage is more severe, some or all data can often be recovered by professional data recovery specialists. However, other RAID levels (like RAID level 5) present much more formidable obstacles to data recovery.
DRIVE ERROR RECOVERY ALGORITHMS
Many modern drives have internal error recovery algorithms that can take upwards of a minute to recover and re-map data that the drive fails to read easily. Frequently, a RAID controller is configured to drop a component drive (that is, to assume a component drive has failed) if the drive has been unresponsive for 8 seconds or so; this might cause the array controller to drop a good drive because that drive has not been given enough time to complete its internal error recovery procedure. Consequently, desktop drives can be quite risky when used in a RAID, and so-called enterprise class drives limit this error recovery time in order to obviate the problem.
A fix specific to Western Digital's desktop drives used to be known: A utility called WDTLER.exe could limit a drive's error recovery time; the utility enabled TLER (time limited error recovery), which limits the error recovery time to 7 seconds. Around September 2009, Western Digital disabled this feature in their desktop drives (e.g., the Caviar Black line), making such drives unsuitable for use in a RAID.
However, Western Digital enterprise class drives are shipped from the factory with TLER enabled. Similar technologies are used by Seagate, Samsung, and Hitachi. Of course, for non-RAID usage, an enterprise class drive with a short error recovery timeout that cannot be changed is therefore less suitable than a desktop drive.
In late 2010, the Smartmontools program began supporting the configuration of ATA Error Recovery Control, allowing the tool to configure many desktop class hard drives for use in a RAID.
RECOVERY TIME IS INCREASING
Drive capacity has grown at a much faster rate than transfer speed, and error rates have only fallen a little in comparison. Therefore, larger capacity drives may take hours, if not days, to rebuild. The re-build time is also limited if the entire array is still in operation at reduced capacity. Given a RAID with only one drive of redundancy (RAIDs 3, 4, and 5), a second failure would cause complete failure of the array. Even though individual drives' mean time between failure (MTBF) have increased over time, this increase has not kept pace with the increased storage capacity of the drives. The time to rebuild the array after a single drive failure, as well as the chance of a second failure during a rebuild, have increased over time.
In order to provide the desired protection against physical drive failure, a RAID must be properly set up and maintained by an operator with sufficient knowledge of the chosen RAID configuration, array controller (hardware or software), failure detection and recovery. Unskilled handling of the array at any stage may exacerbate the consequences of a failure, and result in downtime and full or partial loss of data that might otherwise be recoverable.
Particularly, the array must be monitored, and any failures detected and dealt with promptly. Failure to do so will result in the array continuing to run in a degraded state, vulnerable to further failures. Ultimately more failures may occur, until the entire array becomes inoperable, resulting in data loss and downtime. In this case, any protection the array may provide merely delays this.
The operator must know how to detect failures or verify healthy state of the array, identify which drive failed, have replacement drives available, and know how to replace a drive and initiate a rebuild of the array.
In order to protect against such issues and reduce the need for direct onsite monitoring, some server hardware includes remote management and monitoring capabilities referred to as Baseboard Management, using the Intelligent Platform Management Interface. A server at a remote site which is not monitored by an onsite technician can instead be remotely managed and monitored, using a separate standalone communications channel that does not require the managed device to be operating. The Baseboard Management Controller in the server functions independent of the installed operating system, and may include the ability to manage and monitor a server even when it is in its "powered off / standby" state.
HARWARE LABELING ISSUES
The hardware itself can contribute to RAID array management challenges, depending on how the array drives are arranged and identified. If there is no clear indication of which drive is failed, an operator not familiar with the hardware might remove a non-failed drive in a running server, and destroy an already degraded array.
- A controller may refer to drives by an internal numbering scheme such as 0, 1, 2... while an external drive mounting frame may be labeled 1, 2, 3...; in this situation drive #2 as identified by the controller is actually in mounting frame position #3.
- For large arrays spanning several external drive frames, each separate frame may restart the numbering at 1, 2, 3... but if the drive frames are cabled together, then the second row of a 12-drive frame may actually be drive 13, 14, 15...
- SCSI ID's can be assigned directly on the drive rather than through the interface connector. For direct-cabled drives, it is possible for the drive ID's to be arranged in any order on the SCSI cable, and for cabled drives to swap position keeping their individually-assigned ID, even if the server's external chassis labeling indicates otherwise. Someone unfamiliar with a server's management challenges could swap drives around while the power is off without causing immediate damage to the RAID array, but which misleads other technicians at a later time that are assuming failed drives are in the original locations.
While RAID may protect against physical drive failure, the data is still exposed to operator, software, hardware and virus destruction. Many studies cite operator fault as the most common source of malfunction, such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process. Most well-designed systems include separate backup systems that hold copies of the data, but do not allow much interaction with it. Most copy the data and remove the copy from the computer for safe storage.
Hardware RAID controllers are really just small computers running specialized software. Although RAID controllers tend to be very thoroughly tested for reliability, the controller software may still contain bugs that cause damage to data in certain unforeseen situations. The controller software may also have time-dependent bugs that don't manifest until a system has been operating continuously, beyond what is a feasible time-frame for testing, before the controller product goes to market.