This article is about basic RAID configurations. For RAID in general, see.In, the standard RAID levels comprise a basic set of (redundant array of independent disks) configurations that employ the techniques of, or to create large reliable data stores from multiple general-purpose computer (HDDs). The most common types are RAID 0 (striping), RAID 1 and its variants (mirroring), RAID 5 (distributed parity), and RAID 6 (dual parity). RAID levels and their associated data formats are standardized by the (SNIA) in the Common RAID Disk Drive Format (DDF) standard.While most RAID levels can provide good protection against and recovery from hardware defects or defective sectors/read errors ( hard errors), they do not provide any protection against due to catastrophic failures (fire, water) or soft errors such as user error, software malfunction, or malware infection. For valuable data, RAID is only one building block of a larger data loss prevention and recovery scheme – it cannot replace a plan. Diagram of a RAID 0 setupRAID 0 (also known as a stripe set or striped volume) splits (') data evenly across two or more disks, without information, redundancy,.
Since RAID 0 provides no fault tolerance or redundancy, the failure of one drive will cause the entire array to fail; as a result of having data striped across all disks, the failure will result in total data loss. This configuration is typically implemented having speed as the intended goal. RAID 0 is normally used to increase performance, although it can also be used as a way to create a large logical out of two or more physical disks.A RAID 0 setup can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk.
For example, if a 120 GB disk is striped together with a 320 GB disk, the size of the array will be 120 GB × 2 = 240 GB. However, some RAID implementations allow the remaining 200 GB to be used for other purposes.The diagram in this section shows how the data is distributed into A x stripes on two disks, with A1:A2 as the first stripe, A3:A4 as the second one, etc. Once the stripe size is defined during the creation of a RAID 0 array, it needs to be maintained at all times. Since the stripes are accessed in parallel, an n-drive RAID 0 array appears as a single large disk with a data rate n times higher than the single-disk rate.Performance A RAID 0 array of n drives provides data read and write transfer rates up to n times as high as the individual drive rates, but with no data redundancy. As a result, RAID 0 is primarily used in applications that require high performance and are able to tolerate lower reliability, such as in or.Some benchmarks of desktop applications show RAID 0 performance to be marginally better than a single drive.
Another article examined these claims and concluded that 'striping does not always increase performance (in certain situations it will actually be slower than a non-RAID setup), but in most situations it will yield a significant improvement in performance'. Synthetic benchmarks show different levels of performance improvements when multiple HDDs or SSDs are used in a RAID 0 setup, compared with single-drive performance. However, some synthetic benchmarks also show a drop in performance for the same comparison. Diagram of a RAID 1 setupRAID 1 consists of an exact copy (or ) of a set of data on two or more disks; a classic RAID 1 mirrored pair contains two disks. This configuration offers no parity, striping, or spanning of disk space across multiple disks, since the data is mirrored on all disks belonging to the array, and the array can only be as big as the smallest member disk. This layout is useful when read performance or reliability is more important than write performance or the resulting data storage capacity.The array will continue to operate so long as at least one member drive is operational. Performance Any read request can be serviced and handled by any drive in the array; thus, depending on the nature of I/O load, random read performance of a RAID 1 array may equal up to the sum of each member's performance, while the write performance remains at the level of a single disk.
However, if disks with different speeds are used in a RAID 1 array, overall write performance is equal to the speed of the slowest disk.Synthetic benchmarks show varying levels of performance improvements when multiple HDDs or SSDs are used in a RAID 1 setup, compared with single-drive performance. However, some synthetic benchmarks also show a drop in performance for the same comparison. Diagram of a RAID 2 setupRAID 2, which is rarely used in practice, stripes data at the (rather than block) level, and uses a for. The disks are synchronized by the controller to spin at the same angular orientation (they reach index at the same time ), so it generally cannot service multiple requests simultaneously. However, depending with a high rate, many spindles would operate in parallel to simultaneously transfer data so that 'very high data transfer rates' are possible as for example in the where 32 data bits were transmitted simultaneously.With all hard disk drives implementing internal error correction, the complexity of an external Hamming code offered little advantage over parity so RAID 2 has been rarely implemented; it is the only original level of RAID that is not currently used.
Diagram of a RAID 3 setup of six-byte blocks and two bytes, shown are two blocks of data in different colors.RAID 3, which is rarely used in practice, consists of -level striping with a dedicated disk. One of the characteristics of RAID 3 is that it generally cannot service multiple requests simultaneously, which happens because any single block of data will, by definition, be spread across all members of the set and will reside in the same physical location on each disk. Therefore, any operation requires activity on every disk and usually requires synchronized spindles.This makes it suitable for applications that demand the highest transfer rates in long sequential reads and writes, for example editing. Applications that make small reads and writes from random disk locations will get the worst performance out of this level.The requirement that all disks spin synchronously (in a ) added design considerations to a level that provided no significant advantages over other RAID levels, so it quickly became useless and is now obsolete.
Both RAID 3 and RAID 4 were quickly replaced by RAID 5. RAID 3 was usually implemented in hardware, and the performance issues were addressed by using large disk caches. Diagram 1: A RAID 4 setup with dedicated disk with each color representing the group of blocks in the respective block (a stripe)RAID 4 consists of -level striping with a dedicated disk.
As a result of its layout, RAID 4 provides good performance of random reads, while the performance of random writes is low due to the need to write all parity data to a single disk.In diagram 1, a read request for block A1 would be serviced by disk 0. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently by disk 1. Diagram of a RAID 5 setup with distributed with each color representing the group of blocks in the respective block (a stripe). This diagram shows left asymmetric algorithmRAID 5 consists of block-level striping with distributed parity.
Unlike in RAID 4, parity information is distributed among the drives. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.In comparison to RAID 4, RAID 5's distributed parity evens out the stress of a dedicated parity disk among all RAID members. Additionally, write performance is increased since all RAID members participate in the serving of write requests. Although it won't be as efficient as a striping (RAID 0) setup, because parity must still be written, this is no longer a bottleneck.Since parity calculation is performed on the full stripe, small changes to the array experience write amplification: in the worst case when a single, logical sector is to be written, the original sector and the according parity sector need to be read, the original data is removed from the parity, the new data calculated into the parity and both the new data sector and the new parity sector are written.
This section needs additional citations for. Unsourced material may be challenged and removed.Find sources: – ( January 2015) The following table provides an overview of some considerations for standard RAID levels. In each case:.
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How to plan, manage, and optimize enterprise storage to keep up with the data deluge.RAID 5 works fine when there are no further failures or errors during data reconstruction. Though, almost all SATA drives, and many SCSI drives, were spec'd with one Unrecoverable Read Error (URE) at 10^14.
That's one URE every 12.5TB.One terabyte drives were coming into production then. If you had an 8 drive RAID 5 stripe, and one drive failed, the RAID controller would have to read 7TB of data to reconstruct the failed drive.That meant a better than 50 percent chance that during the reconstruction a URE would scuttle the entire process. When that happens it would have been faster to use a backup to rebuild the data.Of course, drives have only gotten bigger. Four terabyte drives are common and we now have 10TB drives. 1 - 5 of 16It pays to look at spec sheets if you have critical applications or data.
Or you can do what I do. The Storage Bits takeI have a couple of 4 drive RAID 5 arrays. I don't worry about the URE problem because I have all the critical data backed up to the cloud.In case of a drive failure your first action should be to copy all data from the array before replacing the failed drive. If you encounter a URE during copying, at least you've saved all the other data.
Not all low-cost RAID controllers report read errors, so you might copy a corrupted file, but that would have happened anyway.This reiterates the core premise of RAID: it provides data access after drive failures and is NOT a substitute for backup. Fortunately, hard drives are getting more reliable, so your chance of needing this advice is declining.But as drive capacities continue to rise, vendors need to raise their URE spec. When will they do it?Courteous comments welcome, of course. By registering you become a member of the CBS Interactive family of sites and you have read and agree to the,. You agree to receive updates, alerts and promotions from CBS and that CBS may share information about you with our marketing partners so that they may contact you by email or otherwise about their products or services.You will also receive a complimentary subscription to the ZDNet's Tech Update Today and ZDNet Announcement newsletters. You may unsubscribe from these newsletters at any time.ACCEPT & CLOSE.
The reason why RAID 5 might not be reliable for large disk sizes is that statistically, storage devices (even when they are working normally) are not immune to errors. This is what is termed UBE (sometimes URE), for Unrecoverable Bit Error rate, and it is quoted in full-sector errors per number of bytes read. For consumer rotational hard disk drives, this metric is normally specified at 10^-14, meaning that you will get one failed sector read per 10^14 bytes read. (Because of how exponents work, 10^-14 is the same thing as one per 10^14.)10^14 bytes might sound like a big number, but it's really just a handful of full read passes over a modern large (say 4-6 TB) drive. With RAID 5, when one drive fails, there exists no redundancy whatsoever, which means that any error is non-correctable: any problem reading anything from any of the other drives, and the controller (whether hardware or software) won't know what to do. At that point, your array breaks down.What RAID 6 does is add a second redundancy disk to the equation.
This means that even if one drive fails entirely, RAID 6 is able to tolerate a read error on one of the other drives in the array at the same time, and still successfully reconstruct your data. This dramatically reduces the probability of a single problem causing your data to become unavailable, although it doesn't eliminate the possibility; in the case of one drive having failed, instead of one additional drive needing to develop a problem for data to be unrecoverable, now two additional drives need to develop a problem in the same sector for there to be a problem.Of course, that 10^-14 figure is statistical, in the same way as that rotational hard drives commonly have a quoted statistical AFR (Annual Failure Rate) on the order of 2.5%. Which would mean that the average drive should last for 20-40 years; clearly not the case.
Errors tend to happen in batches; you might be able to read 10^16 or 10^17 bytes without any sign of a problem, and then you get dozens or hundreds of read errors in short order.RAID actually makes that latter problem worse by exposing the drives to very similar workloads and environment (temperature, vibration, power impurities, etc.). The situation is worsened further yet by the fact that many RAID arrays are commissioned and set up as a group, which means that by the time the first failure happens, all of the drives in the array will have been active for very near the same amount of time. All this makes correlated failures vastly more likely to happen: when one drive fails, it is very likely to be that case that additional drives are marginal and may fail soon. Merely the stress of the full read pass together with normal user activity may be enough to push an additional drive into failing. As we saw, with RAID 5, with one drive nonfunctional, any read error anywhere else will cause a permanent error and is highly likely to simply bring your array to a halt. With RAID 6, you at least have some margin for further errors during the resilvering process.Because the UBE is stated as per number of bytes read, and number of bytes read tends to correlate fairly well with how many bytes can be stored, what used to be a fine setup with a set of 100 MB drives might be a marginal setup with a set of 1 TB drives and might be completely unrealistic with a set of 4-6 TB drives, even if the physical number of drives remains the same. (In other words, ten 100 MB drives vs ten 6 TB drives.)That is why RAID 5 is generally considered not adequate for arrays of common sizes today, and depending on specific needs RAID 6 or 1+0 is usually encouraged.And that's not even touching on the detail that.
See andFor the calculation of failure RAID, you can use formulas. N is the number of HDD,. p - the probability of failure. q = (1-p) - reliability.The assumption that the probability of failure of the HDD is equal.For clarity, the probability of failure of different RAID at 5 years of work and after it in the table.The probability of failure is RAID DP (Synology) failure of RAID 6.Use p - reliability from Google datacenter search.The probability of failure recovery procedure RAID 5, depending on the capacity.
Answer to your first question. Unrecoverable Read Error.
The disk may be OK, but the data cannot be read preventing rebuild which is the same in the end as a failed disk in terms of a rebuild. I thought the article gave the proper insight on a basic level.Answer to your second question. Same is true for RAID 6 but for larger arrays. I think the point was if you are concerned about a URE for a 12TB array because a spec says you will have 1 URE for every 12TB, then you need an extra redundant disk for every additional 12TB in size to handle all the URE's you should expect to encounter.That is RAID 5 rebuild of 12TB has same chance of failure (per a 10^14 URE rate) as a RAID 6 24TB array. Again, this is extrapolating on the article. The UBE reasoning outlined in the other answers is fine enough, but a greater concern is the risk of a second drive failure during the rebuild.Remember that while the array is being rebuilt the disks are operating at a 100% load, and given the size of modern disks the rebuild can take days. Unless the disks are enterprise grade, they're not really going to like this.
This is the primary reason RAID5 is not suitable for larger disk sizes.You must also consider that when people assemble disk arrays, they usually order the disks from a single vendor. This means that all the disks in the array will be from the same manufacturing batch.
If it's a bad batch, this can mean reduced lifespans, reduced reliability, or even multiple drives failing within a short time period. Even if it's not a bad batch, if the drives begin reaching the end of their lifespan, there's an increased chance that multiple drives will fail within a short time of each other. It's a recommended practice when building an array to split the order up over several vendors, or to ask a single vendor to send you disks from different batches if possible.
This way the drives are more likely to die at different times, and you're unlikely to get multiple drives from a bad batch. Recalls do happen.Look into RAIDZ.
Specifically, look at RAIDZ3, and nested RAIDZ. Synology has something called SynologyHybrid Raid, which has some really nice benefits. You can upgrade the drive sizes in your array just by replacing one drive at a time and waiting for the rebuilds to complete, for example.
Steve Zemanek wrote:Yesterday, a friend and I were having a discussion about Amazon S3's '99% durability' and what it would take to get that type of durability on-premises.I stumbled upon this interesting URL:However, it's calculations are mostly based on hard drive failure rates, and don't take into consideration logical RAID failures. Also, it doesn't calculate RAID 10.But I thought it was interesting.@Steve keep in mind that when AWS and others are talking durability, that is a combination of how the data is protected (e.g. Using different RAID levels, dispersial, erasure or other advanced parity) + multiple versions and copies + in different locations. Thus its not a simple apples to apples comparing basic RAID, or failures of drives or adapters or servers or software etc.However all you need to do is setup a couple of systems that have some level of raid, then have multiples of those systems including in different locations, then place different copies of your data on those systems making sure that you also have multiple versions and you can then get into not just the S3 standard class, you can even then see how 13 or more nines of durable+available are possible. Granted it would be cost effective to setup a bunch of raid 1/10/5/6 or what ever on multiple servers, and then have those in different sites etc.
Steve Zemanek wrote.stumbled upon this interesting URL:Calculations are controversial in storage circles. Absent are differences between consumer, green, and enterprise drives (SATA/SAS, Rotational Vibration Safeguard, Non-recoverable read errors per bits read, etc).Steve Zemanek wrote.Amazon S3's '99% durability'.Typically, Amazon, Google, Facebook, Apple, and others running huge datacenters use much custom backend bits. Traditional RAID is out the window.Dreamhost has some information on its 'dream objects' and Ceph through its website.
Amazon has s3fs, a FUSE based filesystem. Very interesting discussions come up when have a few CS PhDs around. Greg schulz wrote:Steve Zemanek wrote:Yesterday, a friend and I were having a discussion about Amazon S3's '99% durability' and what it would take to get that type of durability on-premises.I stumbled upon this interesting URL:However, it's calculations are mostly based on hard drive failure rates, and don't take into consideration logical RAID failures. Also, it doesn't calculate RAID 10.But I thought it was interesting.@Steve keep in mind that when AWS and others are talking durability, that is a combination of how the data is protected (e.g.
Using different RAID levels, dispersial, erasure or other advanced parity) + multiple versions and copies + in different locations. Thus its not a simple apples to apples comparing basic RAID, or failures of drives or adapters or servers or software etc.However all you need to do is setup a couple of systems that have some level of raid, then have multiples of those systems including in different locations, then place different copies of your data on those systems making sure that you also have multiple versions and you can then get into not just the S3 standard class, you can even then see how 13 or more nines of durable+available are possible. Granted it would be cost effective to setup a bunch of raid 1/10/5/6 or what ever on multiple servers, and then have those in different sites etc. I understand, but we were just trying to make a comparison to standard RAID levels. Robert5205 wrote:Hmmm. Plugged in numbers.one RAID5 failure every 750 years. I can live with that!The problem with this calculator, is it determines odds based on HDD size, speed, rebuild time, and time to replace a failed drive. It does not take into consideration logical failures in the RAID array.
There is a possibility for a software glitch to cause a RAID failure when none of the drives have physically died. RAID 5 is very susceptible to this because of how it uses parity.So although on paper, it might seem like a 1:750 chance in failing, in the real world, it might be much higher. Those calculations are based purely on the MTBF of individual drives and RAID array rebuild time.RAID 5 is dangerous, not so much because of the odds of physical hard drive failures, but RAID 5 is prone to some logical failures because of the way it handles errors and the way it uses parity.SAM explained this pretty well here:I kinda like to just use RAID 1 whenever possible, it's not the highest performance, but it's the simplest. If you need to read data off a drive from a dead server it's probably the easiest to pull data off.
Greg schulz wrote:@Harry good point, I have some drives with a couple of years warranties, however there MTBF is in the millions of hours, or with low (good) AFR numbers. Otoh there are some manufactuers offering 5 year warranties however not all OEMs or vendors offer the extend warranty, seems people prefer the option of saving a few dollars up front on the cost of the drive vs. Having the longer warranty.Hard Drives haven't been following the same Moore's law type trends recently as other parts. I used to always think it was funny that most memory manufacturers offer a 'lifetime' warranty.
But usually memory is so cheap it's not worth the time to file a warranty claim.It's been quite a while since I filed a warranty claim for a hard drive. It's usually not worth the time for the replacement cost.. Steve Zemanek wrote:It's been quite a while since I filed a warranty claim for a hard drive. It's usually not worth the time for the replacement cost.@Steve, cant say that I have ever filed a warranty claim for a HDD or for that matter SSD, maybe its time to look for one that I can try and experience something new with?;) As for HDD not following moore's law, can you show me an example of things that have stuck to moore's law as of late?
As for future HDD growth resuming back at the pace it was in the 2000s, stay tuned, you might be surprised with whats in store (pun intended).
This article is about the data storage technology. For the police unit, see. For other uses, see.RAID ( Redundant Array of Inexpensive Disks or Drives, or Redundant Array of Independent Disks) is a data technology that combines multiple physical components into one or more logical units for the purposes of, performance improvement, or both. This was in contrast to the previous concept of highly reliable mainframe disk drives referred to as 'single large expensive disk' (SLED).Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of and performance.
The different schemes, or data distribution layouts, are named by the word 'RAID' followed by a number, for example RAID 0 or RAID 1. Each scheme, or RAID level, provides a different balance among the key goals:,. RAID levels greater than RAID 0 provide protection against unrecoverable read errors, as well as against failures of whole physical drives. Contents.History The term 'RAID' was invented by, and at the in 1987. In their June 1988 paper 'A Case for Redundant Arrays of Inexpensive Disks (RAID)', presented at the conference, they argued that the top performing disk drives of the time could be beaten on performance by an array of the inexpensive drives that had been developed for the growing market. Storage servers with 24 hard disk drives and built-in hardware RAID controllers supporting various RAID levelsA number of standard schemes have evolved. These are called levels.
Originally, there were five RAID levels, but many variations have evolved, notably several and many (mostly ). RAID levels and their associated data formats are standardized by the (SNIA) in the Common RAID Disk Drive Format (DDF) standard: RAID 0 consists of, but no. Compared to a, the capacity of a RAID 0 volume is the same; it is the sum of the capacities of the disks in the set. But because striping distributes the contents of each file among all disks in the set, the failure of any disk causes all files, the entire RAID 0 volume, to be lost. A broken spanned volume at least preserves the files on the unfailing disks.
The benefit of RAID 0 is that the of read and write operations to any file is multiplied by the number of disks because, unlike spanned volumes, reads and writes are done, and the cost is complete vulnerability to drive failures. RAID 1 consists of data mirroring, without parity or striping. Data is written identically to two drives, thereby producing a 'mirrored set' of drives.
Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first (depending on its and ), improving performance. Sustained read throughput, if the controller or software is optimized for it, approaches the sum of throughputs of every drive in the set, just as for RAID 0. Actual read throughput of most RAID 1 implementations is slower than the fastest drive. Write throughput is always slower because every drive must be updated, and the slowest drive limits the write performance. The array continues to operate as long as at least one drive is functioning. RAID 2 consists of bit-level striping with dedicated parity.
All disk spindle rotation is synchronized and data is such that each sequential is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive. This level is of historical significance only; although it was used on some early machines (for example, the CM-2), as of 2014 it is not used by any commercially available system. RAID 3 consists of byte-level striping with dedicated parity.
All disk spindle rotation is synchronized and data is striped such that each sequential is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.
Although implementations exist, RAID 3 is not commonly used in practice. RAID 4 consists of block-level striping with dedicated parity. This level was previously used by, but has now been largely replaced by a proprietary implementation of RAID 4 with two parity disks, called. The main advantage of RAID 4 over RAID 2 and 3 is I/O parallelism: in RAID 2 and 3, a single read I/O operation requires reading the whole group of data drives, while in RAID 4 one I/O read operation does not have to spread across all data drives.
As a result, more I/O operations can be executed in parallel, improving the performance of small transfers. RAID 5 consists of block-level striping with distributed parity. Unlike RAID 4, parity information is distributed among the drives, requiring all drives but one to be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks. Like all single-parity concepts, large RAID 5 implementations are susceptible to system failures because of trends regarding array rebuild time and the chance of drive failure during rebuild (see 'Increasing rebuild time and failure probability' section, below). Rebuilding an array requires reading all data from all disks, opening a chance for a second drive failure and the loss of the entire array.
In August 2012, Dell posted an advisory against the use of RAID 5 in any configuration on Dell EqualLogic arrays and RAID 50 with 'Class 2 7200 RPM drives of 1 TB and higher capacity' for business-critical data. RAID 6 consists of block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore. RAID 6 requires a minimum of four disks.
As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced. With a RAID 6 array, using drives from multiple sources and manufacturers, it is possible to mitigate most of the problems associated with RAID 5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID 6 instead of RAID 5. RAID 10 also minimizes these problems. Nested (hybrid) RAID. Main article: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 arrays themselves. Arrays are rarely nested more than one level deep.The final array is known as the top array. When the top array is RAID 0 (such as in RAID 1+0 and RAID 5+0), most vendors omit the '+' (yielding and RAID 50, respectively). RAID 0+1: creates two stripes and mirrors them. If a single drive failure occurs then one of the stripes has failed, at this point it is running effectively as RAID 0 with no redundancy. Significantly higher risk is introduced during a rebuild than RAID 1+0 as all the data from all the drives in the remaining stripe has to be read rather than just from one drive, increasing the chance of an unrecoverable read error (URE) and significantly extending the rebuild window. RAID 1+0: (see: ) 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 N+N: With JBOD ( just a bunch of disks), it is possible to concatenate disks, but also volumes such as RAID sets. With larger drive capacities, write delay and rebuilding time increase dramatically (especially, as described above, with RAID 5 and RAID 6). By splitting a larger RAID N set into smaller subsets and concatenating them with linear JBODwrite and rebuilding time will be reduced.
If a hardware RAID controller is not capable of nesting linear JBOD with RAID N, then linear JBOD can be achieved with OS-level software RAID in combination with separate RAID N subset volumes created within one, or more, hardware RAID controller(s). Besides a drastic speed increase, this also provides a substantial advantage: the possibility to start a linear JBOD with a small set of disks and to be able to expand the total set with disks of different size, later on (in time, disks of bigger size become available on the market). There is another advantage in the form of disaster recovery (if a RAID N subset happens to fail, then the data on the other RAID N subsets is not lost, reducing restore time). Non-standard levels. Main article:Many configurations other than the basic numbered RAID levels are possible, and many companies, organizations, and groups have created their own non-standard configurations, in many cases designed to meet the specialized needs of a small niche group. Such configurations include the following:.
provides a general RAID driver that in its 'near' layout defaults to a standard RAID 1 with two drives, and a standard RAID 1+0 with four drives; however, it can include any number of drives, including odd numbers. With its 'far' layout, MD RAID 10 can run both striped and mirrored, even with only two drives in f2 layout; this runs mirroring with striped reads, giving the read performance of RAID 0. Regular RAID 1, as provided by, does not stripe reads, but can perform reads in parallel. has a RAID system that generates a parity file by xor-ing a stripe of blocks in a single HDFS file., the parallel file system, has internal striping (comparable to file-based RAID0) and replication (comparable to file-based RAID10) options to aggregate throughput and capacity of multiple servers and is typically based on top of an underlying RAID to make disk failures transparent.Implementations The distribution of data across multiple drives can be managed either by dedicated.
A software solution may be part of the operating system, part of the firmware and drivers supplied with a standard drive controller (so-called 'hardware-assisted software RAID'), or it may reside entirely within the hardware RAID controller.Hardware-based. See also:Software-implemented RAID is not always compatible with the system's boot process, and it is generally impractical for desktop versions of Windows. However, hardware RAID controllers are expensive and proprietary. To fill this gap, inexpensive 'RAID controllers' were introduced that do not contain a dedicated RAID controller chip, but simply a standard drive controller chip with proprietary firmware and drivers. During early bootup, the RAID is implemented by the firmware and, once the operating system has been more completely loaded, the drivers take over control.
Consequently, such controllers may not work when driver support is not available for the host operating system. An example is, implemented on many consumer-level motherboards.Because some minimal hardware support is involved, this implementation is also called 'hardware-assisted software RAID', 'hybrid model' RAID, or even 'fake RAID'. If RAID 5 is supported, the hardware may provide a hardware XOR accelerator. An advantage of this model over the pure software RAID is that—if using a redundancy mode—the boot drive is protected from failure (due to the firmware) during the boot process even before the operating systems drivers take over. Integrity (referred to in some environments as patrol read) involves periodic reading and checking by the RAID controller of all the blocks in an array, including those not otherwise accessed. This detects bad blocks before use.
Data scrubbing checks for bad blocks on each storage device in an array, but also uses the redundancy of the array to recover bad blocks on a single drive and to reassign the recovered data to spare blocks elsewhere on the drive.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 eight 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, using consumer-marketed drives with RAID can be risky, and so-called 'enterprise class' drives limit this error recovery time to reduce risk. Western Digital's desktop drives used to have a specific fix. A utility called WDTLER.exe limited a drive's error recovery time. The utility enabled, which limits the error recovery time to seven 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 RAID configurations.
However, Western Digital enterprise class drives are shipped from the factory with TLER enabled. Similar technologies are used by Seagate, Samsung, and Hitachi. 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 program began supporting the configuration of ATA Error Recovery Control, allowing the tool to configure many desktop class hard drives for use in RAID setups.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.An array can be overwhelmed by catastrophic failure that exceeds its recovery capacity and the entire array is at risk of physical damage by fire, natural disaster, and human forces, however backups can be stored off site. An array is also vulnerable to controller failure because it is not always possible to migrate it to a new, different controller without data loss. Weaknesses Correlated 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 the assumptions of independent, identical rate of failure amongst drives; failures are in fact statistically correlated. In practice, the chances for a second failure before the first has been recovered (causing data loss) are higher than the chances for random failures.
In a study of about 100,000 drives, the probability of two drives in the same cluster failing within one hour was four times larger than predicted by the —which characterizes processes in which events occur continuously and independently at a constant average rate. The probability of two failures in the same 10-hour period was twice as large as predicted by an exponential distribution. Unrecoverable read errors during rebuild Unrecoverable read errors (URE) present as sector read failures, also known as latent sector errors (LSE). The associated media assessment measure, unrecoverable bit error (UBE) rate, is typically guaranteed to be less than one bit in 10 15 for enterprise-class drives (, or SATA), and less than one bit in 10 14 for desktop-class drives (IDE/ATA/PATA or SATA). Increasing drive capacities and large RAID 5 instances have led to the maximum error rates being insufficient to guarantee a successful recovery, due to the high likelihood of such an error occurring on one or more remaining drives during a RAID set rebuild.
When rebuilding, parity-based schemes such as RAID 5 are particularly prone to the effects of UREs as they affect not only the sector where they occur, but also reconstructed blocks using that sector for parity computation.Double-protection parity-based schemes, such as RAID 6, attempt to address this issue by providing redundancy that allows double-drive failures; as a downside, such schemes suffer from elevated write penalty—the number of times the storage medium must be accessed during a single write operation. Schemes that duplicate (mirror) data in a drive-to-drive manner, such as RAID 1 and RAID 10, have a lower risk from UREs than those using parity computation or mirroring between striped sets., as a background process, can be used to detect and recover from UREs, effectively reducing the risk of them happening during RAID rebuilds and causing double-drive failures.
The recovery of UREs involves remapping of affected underlying disk sectors, utilizing the drive's sector remapping pool; in case of UREs detected during background scrubbing, data redundancy provided by a fully operational RAID set allows the missing data to be reconstructed and rewritten to a remapped sector. Increasing rebuild time and failure probability 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, during which time other drives may fail or yet undetected read errors may surface. The rebuild time is also limited if the entire array is still in operation at reduced capacity.
Given an array with only one redundant drive (which applies to RAID levels 3, 4 and 5, and to 'classic' two-drive RAID 1), a second drive failure would cause complete failure of the array. Even though individual drives' (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.Some commentators have declared that RAID 6 is only a 'band aid' in this respect, because it only kicks the problem a little further down the road. However, according to the 2006 study of Berriman et al., the chance of failure decreases by a factor of about 3,800 (relative to RAID 5) for a proper implementation of RAID 6, even when using commodity drives. Nevertheless, if the currently observed technology trends remain unchanged, in 2019 a RAID 6 array will have the same chance of failure as its RAID 5 counterpart had in 2010.Mirroring schemes such as RAID 10 have a bounded recovery time as they require the copy of a single failed drive, compared with parity schemes such as RAID 6, which require the copy of all blocks of the drives in an array set. Triple parity schemes, or triple mirroring, have been suggested as one approach to improve resilience to an additional drive failure during this large rebuild time.
Atomicity: including parity inconsistency due to system crashes A system crash or other interruption of a write operation can result in states where the parity is inconsistent with the data due to non-atomicity of the write process, such that the parity cannot be used for recovery in the case of a disk failure (the so-called RAID 5 write hole). The RAID write hole is a known data corruption issue in older and low-end RAIDs, caused by interrupted destaging of writes to disk. The write hole can be addressed with. Recently fixed it by introducing a dedicated journaling device (to avoid performance penalty, typically, and are preferred) for that purpose.This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features. Database researcher wrote 'Update in Place is a Poison Apple' during the early days of relational database commercialization. Write-cache reliability There are concerns about write-cache reliability, specifically regarding devices equipped with a, which is a caching system that reports the data as written as soon as it is written to cache, as opposed to when it is written to the non-volatile medium.
If the system experiences a power loss or other major failure, the data may be irrevocably lost from the cache before reaching the non-volatile storage. For this reason good write-back cache implementations include mechanisms, such as redundant battery power, to preserve cache contents across system failures (including power failures) and to flush the cache at system restart time. See also. (NAS).References.
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