raid 5 | DataRecoveryUnion.com

I am in the process of building a new PC and since my data is important. I am considering using RAID. I currently have an external HD which is being backed up using Norton Ghost, but I would feel much more comfortable with real-time protection I’ve read that with on-board RAID controllers, the performance hit for RAID 5 is enormous, so I’m leaning towards RAID 1. I will be using WD 750 Gb Black hard discs on a GIGABYTE GA-P55A-UD4P motherboard using Windows 7. Is the only disadvantage to RAID 1, the ‘loss’ of a hard disc and a slightly more complicated O/S install with the advantages of data protection and a potentially slightly better read performance?

The big advantage or RAID1 is “instant recovery” from HDD failure. That is, if a member of the array fails, the RAID system immediately should detect that situation and convert the operation to using only the remaining good drive so that you can keep on functioning normally right away. It should also immediately send out a warning message so that you know of the problem and can plan its repair as soon as possible. The “downside” of this is that it can work so smoothly that the warning message goes un-noticed or is ignored by untrained users and the tech guys are unaware a problem needs attention. That’s probably not your situation.

The RAID1 systems I have used have very good tools for fixing a drive failure. Basically they will pinpoint exactly which drive is faulty so you can replace it. Then they will allow you to control re-establishing the array by copying everything from the good drive to the replacement unit. There is no need to re-install an OS or restore data from a backup dataset. They even can do this while the system is in use, although my preference would be to do the re-establishment as a separate operation on a system that is NOT being used for anything at the time.

My wife runs a retail store with a POS software package on a dedicated computer. The data files for that operation are kept in one subdirectory and amount to about 60 to 70 MB of data that are updated with every sale. The files are generally in ASCII character strings with some numerical data, so they compress well to .zip files. I set up the machine with a pair of drives in RAID1 as the only drive system. I installed WnZip Pro and set up a scheduled task that runs every day at 10 minutes before midnight (store is closed). It zips all the files in the specific subdirectory into a daily .zip file named with a date string and puts them in a designated subdirectory. This guards against data file corruption by providing end-of-day archived versions. Once a month (probably should be more often) I simply copy the end-of-month .zip file to a USB drive and take it home where I put it on my home computer – thus an off-site backup monthly. Then I delete all the daily .zips at the store, except for that month-end one. (So the store computer has on its RAID1 array an end-of-month .zip file (for every month since its start), each containing a snapshot of all the data that changes over time.) Small important step: the POS computer normally runs 24/7, so when I do the monthly .zip file copy I also reboot the machine and watch the POST messages to be sure there are no errors in the RAID system that I have not heard about.

We had a failure, but not of a hard drive. The mobo failed and had to be replaced. That can be a big problem with any RAID array based on mobo built-in “controllers” because there is no real universal RAID standard. That means often a RAID array written in one system cannot be read by another. In choosing the original mobo (by Abit) I deliberately chose one that had an nVidia chipset because their website claimed that they guarantee that ALL of their mobo chipset RAID systems use the same RAID algorithms and would continue to do so, so that any yet-to-come nVidia chipset could handle any older RAID disks made with their chips. When the mobo failed I selected a Gigabyte replacement mobo with a similar (but not identical) nVidia mobo chipset. Swapped everything, plugged it all together, and booted expecting maybe I’d have to do a Repair Install at least. It just booted and ran perfectly first time – no trouble at all! WOOHOO! I never had to reconfigure or re-install anything, other than updating the mobo device drivers from the Gigabyte CD.

So if you plan for possible changes to the RAID controller system as well as for changes to a hard drive that fails, a RAID1 system can give you some data security and continued operation through disk failure. You just have to recognize the need for real data backups and do them (AND VERIFY, as you say), probably more often than I do it.

Hard Disk Repair

Which RAID Mode Should You Choose?

DATARECOVERYUNION.COM15 years ago4 years ago03 mins

1. Speed (RAID 0)

Set in high-performance mode (also called striped mode or RAID 0) the storage system gives you the power you need when you’re:

Designing huge graphics and need a lightning-fast Photoshop scratch space.
Recording large DV files while maintaining clean audio performance.
Editing DV or HD video and want a smooth work flow with no dropped frames.
Rendering complex 3D objects or special effects.
Performing disk-intensive database operations.
Driven to be the first geek on your block with a computer so fast it blows your
socks off.

Why is RAID 0 so fast? It’s a bit complicated, but suffice it to say that two or more heads, or in this case, drives, are better than one. Picture multiple hoses filling a bucket at the same time or several men bailing a boat and you can understand why two drives striped are

faster

than one. Data is saved (striped) across both drives and accessed in parallel by all the drives so you get

higher data transfer rates

on large data accesses and

higher input/output rates

on small data accesses.

2. Data protection (RAID 1)

Set the system to data protection mode (also known as mirrored mode or RAID 1) and the capacity is divided in half. Half of the capacity is used to store your data and half is used for a duplicate copy.

Why do I want that kind of redundancy? It’s your data, your family pictures, your movie of baby’s first steps, your first novel. Is it important? You decide. If it is, then RAID mirroring is for you.

3. Data protection and speed (RAID 5)

In systems with three or more drives. we recommend that you set the system to RAID 5. This gives you the best of both worlds: fast performance by striping data across all drives; data protection by dedicating a quarter of each drive to fault tolerance leaving three quarters of the system capacity available for data storage.

Data Storage

RAID 6 — Do you really like it?

DATARECOVERYUNION.COM15 years ago4 years ago214 mins

For several years now RAID 5 has been one of the most popular RAID implementations. Just about every vendor that supplies storage to enterprise data centers offers it, and in many cases it has become — deservedly – a well-trusted tool. RAID 5 stripes parity and blocks of data across all disks in the RAID set. Even though users now must devote about 20% of their disk space to the parity stripe, and even though read performance for large blocks may be somewhat diminished and writes may be slower due to the calculations associated with the parity data, few managers have questioned RAID 5’s usefulness.

There are however, two major drawbacks associated with using RAID 5. First, while it offers good data protection because it stripes parity information across all the discs within the RAID set, it also suffers hugely from the fact that should a single disc within the RAID set fail for any reason, the entire array becomes vulnerable — lose a second disc before the first has been repaired and you lose all your data, irretrievably.

This leads directly to the second problem. Because RAID 5 offers no protection whatsoever once the first disc has died, IT managers using that technology have faced a classic Hobson’s choice when they lose a disc in their array. The choices are these. Do they take the system off-line, making the data unavailable to the processes that require it? Do they rebuild the faulty drive while the disc is still online, imposing a painful performance hit on the processes that access it? Or, do they take a chance, hold their breath, and leave the drive in production until things slow down during the third shift when they can bring the system down and rebuild it without impacting too many users?

This choice, however, is not the problem, but the problem’s symptom.

The parity calculations for RAID 5 are quite sophisticated and time consuming, and they must be completely redone when a disk is rebuilt. But it’s not the sophistication of all that math that drags out the process, but the fact that when the disk is rebuilt, parity calculations must be made for every block on the disk, whether or not those blocks actually contained data before the problem occurred. In every sense, the disk is rebuilt from scratch.

An unfortunate and very dirty fact of life about RAID 5 is that if a RAID set contains, say, a billion sectors spread over the array, the demise of even a single sector means the whole array must be rebuilt. This wasn’t much of a problem when disks were a few gigabytes in size. Obviously though, as disks get bigger more blocks must be accounted for and more calculations will be required. Unfortunately, using present technology RAID recovery speed is going to be constant irrespective of drive size, which means that rebuilds will get slower as drives get larger. Already that problem is becoming acute. With half-terabyte disks becoming increasingly common in the data center, and with the expected general availability of terabyte-sized disks this fall, the dilemma will only get worse.

The solution offered by most vendors is RAID 6.

The vendors would have you believe that RAID 6 is like RAID 5 on steroids: it eliminates RAID 5’s major drawback – the inability to survive a second disk failure – by providing a second parity stripe. Using steroids of course comes with its own set of problems.

RAID 6 gives us a second parity stripe. The purpose of doing all of the extra math to support this dual parity is that the second parity stripe operates as a “redundancy” or high availability calculation, ensuring that even if the parity data on the bad disk is lost, the second parity stripe will be there to ensure the integrity of the RAID set. There can be no question that this works. Buyers should, however, question whether or not this added safety is worth the price.

Consider three issues. RAID 6 offers significant added protection, but let’s also understand how it does what it does, and what the consequences are. RAID 6’s parity calculations are entirely separate from the ones done for the RAID 5 stripe, and go on simultaneously with the RAID 5 parity calculations. This calculation does not protect the original parity stripe, but rather, creates a new one. It does nothing to protect against first disk failure.

Because calculations for this RAID 6 parity stripe are more complicated than are those for RAID 5, the workload for the processor on the RAID controller is actually somewhat more than double. How much of a problem that turns out to be will depend on the site and performance demands of the application being supported. In some cases the performance hit will be something sites will live with, however grudgingly. In other cases, the tolerance for slower write operations will be a lot lower. Buyers must balance the increased protection against the penalty of decreased performance.

Issue two has to do with the nature of RAID 5 and RAID 6 failures.

The most frequent cause of a RAID 5 failure is that a second disk in the RAID set fails during reconstruction of a failed drive. Most typically this will be due to either media error, device error, or operator error during the reconstruction – should that happen, the entire reconstruction fails. With RAID 6, after the first device fails the device is running as a RAID 5, deferring but not removing the problems associated with RAID 5. When it is time to do the rebuild, all the RAID 5 choices and rebuild penalties remain. While RAID 6 adds protection, it does nothing to alleviate the performance penalty imposed during those rebuilds.

Need a more concrete reason not to accept RAID 6 at face value as the panacea your vendor says it is? Try this.

When writing a second parity stripe, we of course lose about the same amount of disk space as we did when writing the first (assuming the same number of disks are in each RAID group). This means that when implementing RAID 6, we are voluntarily reducing disk storage space to about 60% of purchased capacity (as opposed to 80% with RAID 5). The result: in order to meet anticipated data growth, in a RAID 6 environment we must always buy added hardware.

This is the point at which many readers will sit back in their chairs and say to themselves, “So what? Disks are cheap!” And so they are — which naturally is one of the reasons storage administrators like them so much. But what if my reader is not in storage administrator? What if the reader is a data center manager, or an MIS director, or a CIO, or a CFO? In other words, what if my reader is as interested in operational expenditures as in the CAPEX?

In this case, the story becomes significantly different. Nobody knows exactly what the relationship between CAPEX and OPEX is in IT, but a rule of thumb seems to be that when it comes to storage hardware the OPEX will be 4-8 times the cost of the equipment itself. As a result, everybody has an eye on the OPEX. And these days we all know that a significant part of operational expenditures derives from the line items associated with data center power and cooling.

Because of the increasing expense of electricity, such sites are on notice that they will have to make do with what they already have when it comes to power consumption. Want to add some new hardware? Fine, but make sure it is more efficient than whatever it replaces.

When it comes to storage, I’m quite sure that we will see a new metric take hold. In addition to existing metrics for throughput and dollars-per-gigabyte, watts-per-gigabyte is something on which buyers will place increased emphasis. That figure, and not the cost of the disk, will be a repetitive expense that managers will have to live with for the life of whatever hardware they buy.

If you’re thinking of adding RAID 6 to your data protection mix, consider the down-stream costs as well as the product costs.

Does RAID 6 cure some problems? Sure, but it also creates others, and there are alternatives worth considering. One possibility is a multilevel RAID combining RAID 1 (mirroring) and RAID 0 (striped parity), usually called either RAID 10 or RAID 1+0. Another is the “non-traditional” RAID approach offered by vendors who build devices that protect data rather than disks. In such cases, RAID 5 and 6 would have no need for all those recalculations required for the unused parts of the disk during a rebuild.

Data Storage

the advantages and disadvantages of RAID 5E

DATARECOVERYUNION.COM15 years ago4 years ago14 mins

RAID 5E with an E that stands for Enhanced, RAID 5E is a RAID 5 array with a hot spare drive that is actively used in the array operations. In a traditional RAID 5 configuration with a hot spare, the hot spare drive sits next to the array waiting for a drive to fail, at which point the hot spare is made available and the array rebuilds the data set with the new hardware. There are some advantages to this operational method:

You know for a fact that the drive that would have been used as a hot spare is in working order.
There is an additional drive included in the array, thus further distributing the array’s I/O load. More spindles equals better performance in most cases. RAID 5E can perform better than typical RAID 5.

There are a few disadvantages associated with RAID 5E as well:

There is not wide controller support for RAID 5E.
A hot spare drive cannot be shared between arrays.
Rebuilds can be slow.

The capacity of a RAID 5E array is exactly the same as the capacity of a RAID 5 array that contains a hot spare. In such a scenario, you would “lose” two disks’ worth of capacity — one disk’s worth for parity and another for the hot spare. Due to this fact, RAID 5E requires that you use a minimum of four drives, and up to eight or 16 drives can be supported in a single array, depending on the controller. The main difference between RAID 5 and RAID 5E is that the drive that would have been used as a hot spare in RAID 5 cannot be shared with another RAID 5 array; so that could affect the total amount of storage overhead if you have multiple RAID 5 arrays on your system. Figure A gives you a look at a RAID 5E array consisting of five drives. Take note that the “Empty” space in this figure is shown at the end of the array.

A RAID 5E array with five drives

When a drive in a RAID 5E array fails, the data that was on the failed drive is rebuilt into the empty space at the end of the array, as shown in Figure B. When the failed drive is replaced, the array is once again expanded to return the array to the original state.

A RAID 5E array that has been rebuilt into the hot spare space

Data Recovery

RAID Recovery – Don’t Increase the Level of Difficulty

DATARECOVERYUNION.COM15 years ago4 years ago04 mins

More and more enthusiast users encounter the destroyed RAID arrays. Generally, data recovery from such a RAID array is possible, but keep in mind that the effort increases disproportionately. First of all, data has to be copied from a RAID drive onto a server, and the data set has to be put back together. The distribution of data into smaller blocks across one or more drives makes RAID 0 the worst possible type to recover. Increasing performance doesn’t necessarily do your data any good here! If a drive is completely defective, only small files, which ended up on only one of the RAID drives (despite the RAID stripe set), can be recovered (at 64 kB stripe size or smaller). RAID 5 offers parity data, which can be used for recovery as well.

RAID data configuration is almost always proprietary, since all RAID manufacturers set up the internals of their arrays in different ways. However, they do not disclose this information, so recovering from a RAID array failure requires years of experience. Where does one find parity bits of a RAID 5, before or after the payload? Will the arrangement of data and parity stay the same or will it cycle? This knowledge is what you are paying for.

Instead of accessing drives on a controller level, the file system level (most likely NTFS) is used, as logical drives will provide the basis for working on a RAID image. This allows the recovery specialist to put together bits and bytes after a successful recovery using special software. The recovery of known data formats is an important approach in order to reach towards a complete data recovery. Take a JPEG file for example – will you be able to recognize a picture after recovery? Or will you be able to open Word.exe, which is found on almost every office system? The selected file should be as large as possible, so it was distributed across all drives and you can know for sure that its recovery was successful.

Two dead hard drives in a RAID 5 are more likely to be restored than two single platters, since RAID still provides parity data.

Data Recovery

RAID Data Recovery Is Possible!

DATARECOVERYUNION.COM15 years ago4 years ago14 mins

What if your hard drive decides to enter the Elysian Fields in this very moment? Sure, you could simply get a new hard drive to substitute for the defective one with a quick run to your favorite hardware store. And with last night’s backup you might even reconstruct your installation quickly. But what if you don’t have a backup? The truth to be more like this: many users don’t even have a backup, or it simply is too old and thus useless for recovering any useful files at all. In case of real hard drive damage, only a professional data recovery specialist can help you – say bye-bye to your vacation savings!

Hard drive failure is especially disastrous for smaller companies working with a single server and a single disk, if they do not have a complete and working data backup at hand. The whole situation is even more complicated if the broken hard drive is a member of a RAID array. Neither hard drive failure in RAID 1 nor RAID 5 will result in data loss, since this scenario has been taken care of by the choice of these RAID levels in advance. But the risk of human error increases: self-made data loss occurs if you accidentally substitute the wrong drive in a degraded RAID 5 array (one with a failed hard drive).

But not all hard drives that show failure symptoms are defective. Sometimes, so called “soft errors” can be fixed using data recovery software. But even in this case, you should weigh the risks to see if it makes sense to take care of the problem yourself or get help from professionals. You might not be able to detect a controller failure right away, for example; usually, users assume a problem with the hard drive. Here is our rule of thumb: if you hear clacking sounds in the potentially defective hard drive, or if the computer’s S.M.A.R.T. function indicates an error during the boot process, something is wrong for sure.

What can you do once you know that an important hard drive is definitely broken? Or what happens if you pulled the wrong drive out of the slot while you were desperately trying to save your data? First of all: don’t panic! You need to act systematically and thoughtfully to be successful, as well as to ensure that you spend as little as possible on recovery – costs can hits four digits easily.

Data Recovery

Raid Data Recovery Softwares

DATARECOVERYUNION.COM16 years ago12 years ago05 mins

1. RAID Reconstructor
Company: Runtime Software
Supported RAID Type: RAID 0, RAID 5
Supported Working Modes: Automatic Analysis

Runtime’s RAID Reconstructor will help you recover data from broken: RAID Level 5 Array consisting of 3 to 14 drives, RAID Level 0 Array (Striping) consisting of 2 to 14 drives. Even if you do not know the RAID parameters, such as drive order, block size and direction of rotation, RAID Reconstructor will analyze your drives and determine the correct values. You will then be able to create a copy of the reconstructed RAID in a virtual image (.vim), an image file (.img) or on a physical drive.

2. DiskInternals Raid Recovery
Company: DiskInternals Data Recoverysoftware
Supported RAID Type: RAID 0, 1, JBOD, RAID 5, and 0+1
Supported Working Modes: Automatic Analysis

Recover corrupted RAID arrays in a fully automatic mode. Raid Recovery is the first tool to automatically detect the type of the original RAID array while still allowing for fully manual operation. Raid Recovery is no doubt a highly valuable tool for users of all types of RAID arrays, whether hardware, native, or software. The drag-and-drop user interface allows specifying parts of the RAID array by simply dragging and dropping icons representing the disks.

3. Quick Recovery RAID
Company: Unistal Immortalizing Information
Supported RAID Type: RAID 0, RAID 5
Supported Working Modes: Manual Analysis

Quick Recovery RAID is a do-it-yourself non-destructive raid data recovery software. There are just two steps to perform the complete operation. Analysis, Select & Save. Analysis is the most important aspect of data recovery. Quick Recovery RAID’s unique Guided File Excavation Technology (GFETCh) helps in locating files and folders lost behind overwritten partitions too.

4. RAID Recovery Presentation
Company: R-Studio
Supported RAID Type: RAID 0, RAID 1, RAID 5
Supported Working Modes: Manual Analysis

R-Studio detects and treats valid software or hardware RAIDs as regular drives/volumes. But what to do if you have only drives or drive images of a faulty RAID? R-Studio can still help you to get the data back provided that the drives necessary for the RAID to operate are working or you have the images of those drives. The number of drives enough to get data back depends on the RAID layout. For example, for a mirror (RAID 1) of two drives, at least one must be valid, whereas for a RAID5 of 3 disks, the number of valid drives should be two.

5. RAID recovery
Company: Zero Assumption Recovery
Supported RAID Type: RAID 0, RAID 5
Supported Working Modes: Manual Analysis

This tutorial describes the data recovery procedure used to recover a RAID0 or RAID5 array if the controller failed and the array parameters are lost Windows software-based RAID configuration data is damaged.

6. Getway Raid Recovery
Company: Getway Recovery LTD
Supported RAID Type: RAID 0, Raid 5,Raid 5E,Raid 5EE,Raid 6
Supported Working Modes: Smart Mode,Manual Mode,User-define Mode

Getway Raid Recovery is the professional raid recovery software which can extract data from multiple Hard disks in a RAID system, and rebuild the correct data. It can get data back from various types of arrays, including RAID 0, RAID 5, RAID 5E, RAID 5EE and RAID 6.

Data Recovery

RAID 5 Data Recovery

DATARECOVERYUNION.COM16 years ago4 years ago124 mins

1. RAID 5 Data Recovery FAQ

Q: What is the definition of a “RAID 5” volume?
A: “RAID 5” refers to a “Redundant Array of Inexpensive (or Independent) Disks” that have been established in a Level 5, or striped with parity, volume set. A RAID 5 volume is a combination of hard drives that are configured for data to be written across three (3) or more drives.

Q: What is “parity” or “parity data”?
A: In a RAID 5 configuration, additional data is written to the disk that should allow the volume to be rebuilt in the event that a single drive fails. In the event that a single drive does fail, the volume continues to operate in a “degraded” state (no fault tolerance). Once the failed drive is replaced with a new hard drive (of the same or higher capacity), the “parity data” is used to rebuild the contents of the failed drive on the new one.

Q: What the minimum drive requirements to create a RAID 5 volume?
A: RAID 5 volume sets require a minimum of at least three (3) hard drives (preferably of the same capacity) to create and maintain a RAID 5 volume. If one drive is of a lower capacity than the others, the RAID controller (whether hardware or software) will treat every hard drive in the array as though it were of the same lower capacity and will establish the volume accordingly.

Q: What are the differences between “hardware” and “software” RAID 5 configurations?
A: With a software-based RAID 5 volume, the hard disk drives use a standard drive contoller and a software utility provides the management of the drives in the volume. A RAID 5 volume that relies on hardware for management will have a physical controller (commonly built into the motherboard, but it can also be a stand-alone expansion card) that provides for the reading and writing of data across the hard drives in the volume.

Q: What are the advantages of RAID 5 volumes?
A: A RAID 5 volume provides faster data access and fault tolerance, or protection against one of the drives failing during use. With a RAID 5 disk volume, information is striped (or written) across all of the drives in the array along with parity data. If one of the hard drives in the array becomes corrupted, drops out of a ready state or otherwise fails, the remaining hard drives will continue to operate as a striped volume with no parity and with no loss of data. The failed drive can be replaced in the array with one of equal or larger capacity, and the data it contained will be automatically rebuilt using the parity data contained on the other drives. Establishing a RAID 5 volume requires 3 disk drives as a minimum requirement.

Q: What are the disadvantages of RAID 5 configurations?
A: There are several disadvantages. RAID 5 results in the loss of storage capacity equivalent to the capacity of one hard drive from the volume. For example, three 500GB hard drives added together comprise 1500GB (or roughly about 1.5 terabytes) of storage. If the three (3) 500GB drives were established as a RAID 0 (striped) configuration, total data storage would equal 1500GB capacity . If these same three (3) drives are configured as a RAID 5 volume (striped with parity), the usable data storage capacity would be 1000GB and not 1500GB, since 500GB (the equivalent of one drives’ capacity) would be utilized for parity. In addition, if two (2) or more drives fail or become corrupted at the same
time, all data on the volume would be inaccessible to the user.

Q: Can data be recovered from a re-formatted RAID 5 volume?
A: Many times information is still recoverable, depending on how the drives were re-formatted. Re-formatting a volume using Windows, for example, will create what will appear to be a new “clean” volume – but the original data will still be on the disk in the “free and available” space. However, a low-level format (usually performed through an on-board RAID controller utility) will “wipe” or overwrite every single block on a drive. Unlike an O/S (or “high-level”) format, a low-level format normally is slower, takes a considerable amount of time and destroys the original data.

Q: Can I run recovery software utilities to recover my RAID volume data?
A: The safest approach to data recovery with a RAID volume (or with any media) is to capture every storage block on each device individually. The resulting drive “images” are then used to help rebuild the original array structure and recover the necessary files and folders. This approach limits continued interaction with the media and helps to preserve the integrity of the original device. One of the dangers in using data recovery software is that it forces the read / write heads to travel repeatedly over areas of the original media which, if physically damaged, could become further damaged and possibly unrecoverable.

Q: If a RAID 5 volume will not mount, should I allow a “rebuild” to run?
A: If one drive fails in a RAID 5 configuration, the volume still operates – but in a degraded state (it no longer writes parity information). The important data should be backed up immediately and verified to be usable before any rebuild operation is started. When it comes to critical data, anything that is used to read or write to the original volume represents a risk. Is the hardware operating properly? Are all other drives in the volume functioning correctly? If you are the least bit unsure, a rebuild should not be performed.

Q: If multiple drives fail in a RAID volume all at once, is the data still recoverable?
A: In many cases, the answer is yes. It usually requires that data be recovered from each failed hard drive individually before attempting to address the rest of the volume. The quality and integrity of the data recovered will depend on the extent of the damage incurred to each failed storage device.

2. How Raid 5 Data Recovery?
RAID 5 is a very popular RAID level that uses block level striping and distributed parity. This level tries to remove the bottleneck of the dedicated parity drive. With the use of a distributed parity algorithm, this level writes the data and parity data across all the drives. Basically, the blocks of data are used to create the parity blocks which are then stored across the array. This removes the bottleneck of writing to just one parity drive. However, the parity information still has to be written on a separate disk whenever a write occurs, so the slowdown involved with that still applies. There is also a small calculation that must take place for every write. The fault tolerance is maintained by separating the parity information for a block from the actual data block. This way when one drive fails, the array goes into degraded mode and begins reading and writing to the parity areas on the other disks in place of that bad drive. When a new disk is placed back into the RAID, the controller or software begins copying the parity data back to the new drive until complete, then the array will kick out of degraded mode. Recovery is more complicated than usual because of the distributed nature of the parity. Many RAID cards and software use separate and sometimes proprietary algorithms to generate the parity stripes. On illustration A you see just one example of RAID 5, generally referred to as standard or straight RAID 5. Many times you can get the striping pattern from the RAID card or software manufacturer.

As you can see in the illustration above, there is a clear pattern. The sectors in the virtual disk are striped evenly across the disks, but every fourth stripe is dedicated to parity. Red denotes parity data.

Controller Requirements: Supported by most hardware controllers, both SCSI and IDE/ATA, and also most software RAID solutions.

Hard Disk Requirements: Minimum of three hard. Any type may be used, but they should be of identical type and size for best performance and to eliminate “waste”.

Array Capacity: (Size of Smallest Drive * Number of Drives Smallest Drive).

Fault Tolerance: Any one drive may fail and the array continues to operate (in fact, it operates faster in degraded mode!) Failure of another drive results in loss of all data, which is why you paid the big bucks!

Storage Efficiency: 75% if identical drives are used.
Availability: Loss of one disk = continued server functionality.
Rebuilding (Scrubbing) and Degradation: Rebuilding takes place automatically with most RAID cards and software.
Random Read Performance: Excellent
Random Write Performance: Moderate
Sequential Read Performance: Moderate
Sequential Write Performance: Very good.

RAID 5 uses a distributed parity algorithm, this level writes the data and parity data across all the drives. The blocks of data are used to create the parity blocks which are then stored across the array. Block size can be anything, but is typically 64kB (128 sectors) Disk 0 will contain the first sector 0 through 127, disk 1 will contain sectors 128 through 255, and this will continue to alternate until you reach the last disk of the set, and this disk will be the parity disk. The parity disk will rotate based on the parity rotation algorithm for that particular RAID card or software. One complication can be expected in some cases, and that is the presence of an offset. An offset is a number of sectors before the first striped block. The presence of an offset is common in Adaptec cards. The offset can easily be found by searching for the partition table. When found, simply take the sector number where the partition table is located, and clone the disk to a file starting with this sector. Repeat on all drives and you have a starting point!

The next step is to find the stripe size. This is a very critical step and you must be absolutely sure. Typically the stripe size will be the same as the default setting for the card that was used. For instance, a Dell PERC 2 adaptec RAID card has a stripe size of 32K (64 sectors) and an offset of 64K (128 sectors). Use this as your starting point if possible. If you do not know the card type used, it is wise to use 64K (128 sectors) as your starting point as this is most common among all cards.

Now use Winhex to find a location on the disk that is easy to see a pattern. See the example below. Notice how we have text, apparently from a database of some sort. This text can be used to identify a data pattern. Now look at the current sector (53,721,904). Divide this number by the suspected stripe size in sectors. In this case the stripe size we are attempting to validate is 128 sectors. The resulting number will probably not be a whole number. In this case it is 419702.375. Take the whole number of 419702 and multiply this by the suspected stripe size (128). The resulting number is what we will refer to as the stripe break point. It is necessary to know this simple calculation for all types of RAID except RAID 1 (mirroring).

Find the break point:
53721904/128=419702.375419702*128 = 53721856

Answer: A break point is located at sector 53, 721, 856

Notice above how we have text, apparently from a database of some sort. This text can be used to identify a data pattern.

Notice how at the exact break point of 53, 721, 856 we have a definite difference of data. This is because the stripe is from a separate area of the volume. Not all break points will be this easy. In some cases you will have to look at the actual data and determine if consistency exists. Train your eyes to catch a break point while you are scrolling the sectors using the page down function, and you will become very proficient. You will often have to repeat the steps above on different areas of the disk if the data is too inconsistent to determine the break point.

Once the break point is discovered, you will then be able to start the RAID 5 de-striping process.

The best starting point is to clone all disks twice (to be sure) into image files on separate disks. Obtain the original card or find out the card make and model and purchase this.

Assuming you have no idea where the disks belong in the RAID then you must find a point on the disk where the data is sequential. This is very difficult unless the volume is formatted with NTFS, FAT32, or FAT16. In this case, you can use the Master boot record and NTFS/FAT32/FAT16 boot record to find the location of the MFT files or FAT tables.

RAID-5 Parity Rotation
RAID-5 under any operating system can use one of four algorithms for the placement of segments among the disks in the array. -Keep in mind in your troubleshooting that there may be an offset throwing everything off. Find the partition table or OS identifier and us this as your definite sector 0. In a RAID 5 there should be two drives with a partition table. One is the first drive in that array and one is the last drive in the array.

Right Synchronous
Left Synchronous,
Left Asynchronous
Right Asynchronous

Left Asynchronous (Backwards Parity Rotation, Standard)
In this layout, the segments are numbered sequentially, starting with the first non-parity drive in the stripe. The parity drive starts at the last drive, and moves backwards one drive per stripe. While this is the hardware ‘standard’ RAID-5 layout, it is not the default for Linux or Windows 2000, 2003 Server. This is sometimes called backwards parity or standard Rotation R-studio supports this mode.

Left Synchronous
In this layout, the segments are numbered sequentially, starting with the first drive in the stripe after the parity. The segments wrap. The parity drive starts at the left-most drive, and moves right one drive per stripe. This is the default RAID-5 segment layout under Linux.

For large reads, this segment layout is the fastest. This is because each consecutive group of segments that is no larger than the total number of disks in the array, will use all the disks in the array.

Right Asynchronous (Forward Parity Rotation)
In this layout, the segments are numbered sequentially, starting with the first non-parity drive in the stripe. The parity drive starts at the right-most drive, and moves left one drive per stripe.

Right Synchronous
In this layout, the segments are numbered sequentially, starting with the first drive in the stripe after the parity. The segments wrap. The parity drive starts at the right-most drive, and moves left one drive per stripe.

Refer to the partition and boot sector repair section of this manual if the disk is not mountable, or review the stripe break points.

Recommended RAID 5 Recovery Software: Getway Raid Recovery Software

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter R)

DATARECOVERYUNION.COM16 years ago7 years ago06 mins

RAID
Redundant Array of Independent Disks, a collection of two or more disks working together in an array. Mylex RAID controllers implement this technology to connect up to 15 SCSI devices per channel. The different forms of RAID implementation are known as “RAID levels.” See also Berkeley RAID Levels, Disk Array, and RAID Levels.

The system manager or integrator selects the appropriate RAID level for a system. This decision will be based on which of the following are to be emphasized:

Disk Capacity

Data Availability (redundancy or fault tolerance)

Disk Performance

RAID Adapters
See RAID Controller

RAID Advisory Board (RAB)
An association of companies whose primary intention is to standardize RAID storage systems. Mylex is a member of RAB.

RAID Controller
Low cost RAID controllers that use SCSI channels on the motherboard.

RAID Levels
Mylex disk array controllers support four RAID Advisory Board approved (RAID 0, RAID 1, RAID 3, and RAID 5), two special (RAID 0+1, and JBOD), and three spanned (RAID 10, 30, and 50) RAID levels. All DAC960, AcceleRAID, and eXtremeRAID series controllers support these RAID levels. See also Berkeley RAID Levels.

– Level 0:
Provides block “striping” across multiple drives, yielding higher performance than is possible with individual drives. This level does not provide any redundancy.

– Level 1:
Drives are paired and mirrored. All data is 100 percent duplicated on a drive of equivalent size.

– Level 3:
Data is “striped” across several physical drives. Maintains parity information, which can be used for data recovery.

– Level 5:
Data is “striped” across several physical drives. For data redundancy, drives are encoded with rotated XOR redundancy.

– Level 0+1:
Combines RAID 0 striping and RAID 1 mirroring. This level provides redundancy through mirroring.

– JBOD:
Sometimes referred to as “Just a Bunch of Drives.” Each drive is operated independently like a normal disk controller, or drives may be spanned and seen as a single drive. This level does not provide data redundancy.

– Level 10:
Combines RAID 0 striping and RAID 1 mirroring spanned across multiple drive groups (super drive group). This level provides redundancy through mirroring and better performance than Level 1 alone.

– Level 30:
Data is “striped” across multiple drive groups (super drive group). Maintains parity information, which can be used for data recovery.

– Level 50:
Data is “striped” across multiple drive groups (super drive group). For data redundancy, drives are encoded with rotated XOR redundancy.

Note: The host operating system drivers and software utilities remain unchanged regardless of the level of RAID installed. The controller makes the physical configuration and RAID level implementation.

RAID Migration
A feature in RAID subsystems that allows for changing a RAID level to another level without powering down the system.

Read-Ahead Cache
A caching strategy whereby the computer anticipates data and holds it in cache until requested.

Recovery
The process of reconstructing data from a failed disk using data from other drives.

Redundancy
The inclusion of extra components of a given type in a system (beyond those the system requires to carry out its functions).

Rotated XOR Redundancy
XOR refers to the Boolean “Exclusive-OR” operator. Also known as Parity, a method of providing complete data redundancy while requiring only a fraction of the storage capacity of mirroring. In a system configured under RAID 3 or RAID 5 (which require at least three SCSI drives), all data and parity blocks are divided amongst the drives in such a way that if any single drive is removed (or fails), the data on it can be reconstructed using the data on the remaining drives. In any RAID 3 or RAID 5 array, the capacity allocated to redundancy is the equivalent of one drive.