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CrAsHnBuRnXp Jul 03, 2007, 11:59pm EDT Reply - Quote - Report Abuse
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I have compiled a list of RAID arrays so that people are more aware of what each one does and if it would be useful for them. I will provide advantages and disadvantages of each RAID array given. First, a brief history about RAID.

History of RAID: The idea of RAID (Redundant Array of Inexpensive [or independent] Disks) was designed as a fix to a problem of bestowing a high capacity storage combined with data availability and redundancy. In the past when hard drives capacities were limited and higher capacity drives were expensive, RAID offered little data protection and redundancy. Compounding the problem, CPU processor performance was increasing at an exponential rate, while disk subsystems were quickly falling behind and creating a bottleneck for server performance.

Back in 1988, a few researchers from the University of California-Berkeley came up with a set of guidelines for the original implementation of RAID. These guidelines would be referred to as RAID-1 through RAID-6. The various RAID levels do NOT mean that RAID 6 would be better than say RAID-1 or RAID-5. Your needs will determine what RAID level is best for your current situation.

Now, when using RAID, it’s recommended to use the same size drives. You can in fact, use various size drives in any given array, but the array will take the form of the smallest hard drive and the rest of the unallocated space on the larger hard drive will not be use and in fact just be a waste. For example, if you wanted to setup a RAID-0 array, it's recommended to have a minimum of two drives of the same size such as 2x80GB. Whenever using any RAID array, the array will combine the number of drives you are using and make it one entire drive. So if you were to RAID-0 2x80GB hard drives, this would make a 160GB hdd. (Will be less when you factor in the formatting)

RAID-0 - Data Striping w/out parity– 2 disk minimum:
Provides improved performance to that of a single, non-RAID-0 drive, and provides additional storage space to work with. This RAID array breaks down the information stored on the hard drive into blocks which are stored on each corresponding RAID-0 hard drive.

Array size: Size of Smallest Drive x Number of Drives

Advantages: This particular array is the easiest to implement, cheapest to implement, and most all controllers will support the use of RAID-0. Can make boot times quicker and make applications load faster.

Disadvantages: Not fault tolerant. In other words if one drive fails, all data is lost.

Recommendation: Do not use it in an environment where data is of the utmost importance such as a law firm or school corporation. If you implement this array, it is HIGHLY recommended that you schedule daily or weekly backup. (Preferably every couple of days or whenever you add new data) I would not use more than four drives either because you run the risk of losing data. One is better off to install RAID-0 in an environment that where applications require a high amount of performance such as gaming or working with digital imaging. Backup is required so that way if one (or all) drives fail, you can recover from the failure.

RAID-1 – Mirroring & Duplexing – 2 disk minimum without parity:
Set of two disks or more that more or less mirror one another. Meaning the data being written to the primary disk it is being duplicated on the secondary disk (or all other disks in the array). Data is written to all disks at the same time and can be read from each disk separately. Thus enhancing read time. The transfer rate per written block is equal to that of a single disk. If the primary disk in the array fails, the array can be configured to use the mirrored copy on one of the other disks in the array until you can replace the failed hard drive. After which, the data can be restored into the new drive from the other remaining drives in the array. This is NOT a substitute for backups.

Array size: takes the size of the smallest drive.

Advantages: 100% redundant. In other words if a single drive is lost to a failure, you will not lose data. RAID-1 can withstand multiple drive failures. RAID-1 is another simple array setup to implement.

Disadvantages: One of the lease efficient RAID arrays.

Recommendation: Best used in an environment that requires high read performance such as accounting, company payroll, or financial situations. You are still highly recommended to backup your data.

RAID-2 – Hamming Code ECC – 1 or more disks:
This RAID array performs disk striping at the bit level. The error-checking and correction can only be supported with a certain kind of hard drive. When a hard drive read occurs, the data on the drive is checked with the ECC codes to establish that everything is correct. If it happens to be incorrect, the data is corrected on the “fly”.

Array size: Varies

Advantages: Fault tolerant, “on the fly” data correction, high data transfers, simpler RAID design compared to RAID-3, 4, and 5.

Disadvantages: Not commercially available, high entry level cost, and it requires a high transfer rate.

Recommendation: Best left for business purposes.

RAID-3 – Parallel Transfer (Striping) with Parity – 3 disk minimum:
Data is divided amongst and written to the separate hard drives. The parity is generally made on writes, written to the parity drive, and checked on the read. If a disk happens to fail, then the data is restored across the striped array using the parity information that was written to one of the other hard drives. The performance of the disk reads in RAID-3 is that of a RAID-0 implementation. If you add more drives to increase the total size of the RAID-3 array, then the parity size of the drive must also be increased so that it can match or surpass the physical size of the individual array drives.

Array size: Size of Smallest Drive x Number of Drives - 1

Advantages: Fault tolerant, high read and write of data transfer, disk failure has an exiguous amount of impact, and has a high efficiency.

Disadvantages: Difficult and resource intensive if used in software RAID, complex, and the transaction rate is equal to a single hard drive (so long as the spindles are in sync)

Recommendation: Video production and or live streaming, Editing of Image and Video and any other application requiring high throughput/best for applications that require sequential data reads.

RAID-4 – Independent Data Disks w/ Shared Parity – 3 disk minimum:
It is similar to RAID-3 in that it contains a number of striped disks and it has a separate parity disk. However, the size of the striping block is bigger to reconcile more data. This is what makes RAID-4 similar to RAID-3 in that it has basically the same implementation, but it removes the bottlenecks that affected the transactional data in RAID-3.

Array size: Size of Smallest Drive x Number of Drives – 1

Advantages: High read rate, high aggregate read, Low parity (high efficiency)

Disadvantages: The worst write rate, worst write aggregate rate, difficult to rebuild in the event of a hard drive failure, block read rate is that of a single disk, not commercially available

Recommendation: Not a recommended use. There are better options to choose from.

RAID-5 – Striping with Parity – 3 disk minimum:
This is the most widely used RAID array used today. What RAID-5 does is the parity information gets distributed amongst all drives within the array unlike RAID-3 or 4. A certain amount of total disk space becomes unavailable on the array so that the parity data can be written to disk. Usually, the amount of drive space given for parity information is equal to the size of one entire drive in the array. Example, an array of 4x10GB drives would give you approximately 30GB of space for your data while the left over 10GB would be reserved for the parity information.

Array size: Size of Smallest Drive x Number of Drives - 1

Advantages: Fault tolerant, read speeds are quite high, high efficiency, good transfer rate

Disadvantages: Disk failure has a medium impact on the array (meaning you can only sustain one drive failure at a given time), has the most complex design, difficult to rebuild after a disk failure

Recommendation: File servers, database servers, Web servers, Email servers, Intranet servers, etc.

RAID-6 – Striping with Double Parity – 4 disk minimum plus a proprietary RAID controller:
RAID-6 is the exact same thing as RAID-5, but it offers double the parity of RAID-5 so that way you can sustain a two disk failure and still retain your data.

Array size: Size of Smallest Drive x Number of Drives - 2

Advantages: Fault tolerant, can sustain a two disk failure, perfect for a mission critical environment

Disadvantages: More complex, controller overhead for the parity is very high,

Recommendation: File servers, database servers, Web servers, Email servers, Intranet servers, etc.

Please note that neither RAID array is a preventative from doing regular backups. Backups are still highly recommended in case of an unforeseeable event.


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Adam Kolak Jul 04, 2007, 12:02am EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Nice write-up.

They need to make this a sticky in the Hard Drive forum, but not on the Latest Topics, that's too crowded with stickys.

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Beavis Khan Jul 09, 2007, 10:55am EDT Reply - Quote - Report Abuse
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Edited: Jul 09, 2007, 11:34am EDT

 
>> Re: RAID Arrays Explained
Mark Allen said:
RAID-1 can withstand multiple drive failures.

This is true only if you have more than 2 drives in the array (which would be unusual, but certainly not impossible).

Regarding RAID 5/6, another significant disadvantage is (relatively) lousy write performance. If your application is a write-intensive database, you'd be much better off going with RAID 1, or a hybrid approach (eg RAID 1+0).

It might also be worth mentioning that many "real" RAID controllers will allow you to designate a drive as a "hot spare", meaning that in the event of a member disk failure, the controller will add the hot spare into the array, and begin rebuilding right away. This is usually done only with RAID 5 arrays.

Edit - one more minor nitpick - the title should be "Arrays". You don't need an apostrophe.

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CrAsHnBuRnXp Jul 09, 2007, 01:31pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Big Beavis said:
RAID-1 can withstand multiple drive failures.

This is true only if you have more than 2 drives in the array (which would be unusual, but certainly not impossible).

Regarding RAID 5/6, another significant disadvantage is (relatively) lousy write performance. If your application is a write-intensive database, you'd be much better off going with RAID 1, or a hybrid approach (eg RAID 1+0).

It might also be worth mentioning that many "real" RAID controllers will allow you to designate a drive as a "hot spare", meaning that in the event of a member disk failure, the controller will add the hot spare into the array, and begin rebuilding right away. This is usually done only with RAID 5 arrays.

Edit - one more minor nitpick - the title should be "Arrays". You don't need an apostrophe.

Multiple drive failures meaning that you can only lose one drive multiple times in that array.

Good catch on the "Arrays". I didn't even notice that. I'm not sure why I did that in the first place.

FordGT90Concept Jul 19, 2007, 02:22am EDT Reply - Quote - Report Abuse
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Edited: Jul 19, 2007, 02:24am EDT

 
>> Re: RAID Arrays Explained
RAID0 = Fast but high risk
RAID1 = Slow but low risk
RAID 0+1/10 = Hybrid of RAID0 and RAID1 being about as fast as RAID0 with the redundancy of RAID1 at the expensive of inefficiency (lots of drives required)
RAID5 = Pretty fast with mediocre risk (can lose one drive but no more). RAID5 is XOR mad which, if you don't have a good RAID card, that means a lot more work for the CPU. I would only ever consider RAID5 if you got a good RAID card that can handle it. RAID5 is far more efficient than RAID 0+1/10 in terms of read/write speed and capacity. Should a drive fail, RAID5 takes a long time to rebuild the lost drive.

If you want mad write performance on RAID5, use Write Through (write requests take immediate priority over read requests). If write performance isn't critical, use Write Back (caches all write requests and writes when it can).

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_MD_ Jul 19, 2007, 09:34am EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Adam K. said:
They need to make this a sticky in the Hard Drive forum, but not on the Latest Topics, that's too crowded with stickys.

True that

I had a bad experience with RAIDs... I used to have a SCSI RAID setup and one disk failed. So alright, I insert a new disk, but the crash somehow damaged the RAID controller as well. So I was on the market for a SCSI RAID controller... which was a legacy device (has been used by the company for years) and was not in productions anymore (for a long time actually, so couldn't find even used). So all of the data was lost. Since then, we have a mirrored tape (and now NAS) backup at the end of each day...
Now I know there might be comments about this, but I guess the company is just somewhat skeptic to give RAID another try... lol

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Beavis Khan Jul 23, 2007, 11:45am EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
_MD_ said:
I had a bad experience with RAIDs... I used to have a SCSI RAID setup and one disk failed. So alright, I insert a new disk, but the crash somehow damaged the RAID controller as well. So I was on the market for a SCSI RAID controller... which was a legacy device (has been used by the company for years) and was not in productions anymore (for a long time actually, so couldn't find even used). So all of the data was lost. Since then, we have a mirrored tape (and now NAS) backup at the end of each day...
Now I know there might be comments about this, but I guess the company is just somewhat skeptic to give RAID another try... lol


This is definitely the main pitfall with hardware RAID 5. In fact, if your write throughput requirements aren't too high, I'd generally recommend doing software RAID5 instead, simply because there is no proprietary hardware to fail, and the array can (at least in theory) be used/reconstructed on any computer. Both Linux and Windows server (both 2000 and 2003) include decent tools for managing software-based RAID arrays. You have a lot more flexibility under Linux, but of course, this comes with the price of additional complexity.

Regardless though, RAID is never (never!) a substitute for a robust backup scheme :)

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Lou Bot Jul 23, 2007, 01:06pm EDT Reply - Quote - Report Abuse
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Edited: Jul 23, 2007, 01:09pm EDT

 
>> Re: RAID Arrays Explained
No RAID10? not that it is important but here is more info from wiki website

Nested RAID levels
Main article: Nested RAID levels
Many storage controllers allow RAID levels to be nested. That is, one RAID can use another as its basic element, instead of using physical drives. It is instructive to think of these arrays as layered on top of each other, with physical drives at the bottom.

Nested RAIDs are usually signified by joining the numbers indicating the RAID levels into a single number, sometimes with a '+' in between. For example, RAID 10 (or RAID 1+0) conceptually consists of multiple level 1 arrays stored on physical drives with a level 0 array on top, striped over the level 1 arrays. In the case of RAID 0+1, it is most often called RAID 0+1 as opposed to RAID 01 to avoid confusion with RAID 1. However, when the top array is a RAID 0 (such as in RAID 10 and RAID 50), most vendors choose to omit the '+', though RAID 5+0 is more informative.


[edit] Common nested RAID levels
RAID 0+1: Striped Set + Mirrored Set (4 disk minimum; Even number of disks) 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 two or more drives fail on different sides of the mirroring, the data on the RAID system is lost.
RAID 1+0: Mirrored Set + Striped Set (4 disk minimum; Even number of disks) 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 as long as no two drives lost comprise a single pair of one mirror.
RAID 5+0: A stripe across distributed parity RAID systems
RAID 5+1: A mirror striped set with distributed parity (some manufacturers label this as RAID 53)

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phil Jul 23, 2007, 08:27pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
how about adding what stripe width, size, columns, and are and how changing the stripe size will affect read/write/positioning speeds in different arrays being used, and how they are being used (large database, small file copies, etc)

.. if you'd like some assistance let me know

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Meats_Of_Evil Jul 26, 2007, 05:43pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
I must thank you Mark for this information. I only knew a little about Raid 0 and that's it, but thanks to you I might Raid my Pc just for the heck of it :P

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<a class= Aug 05, 2007, 07:08pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
what about other levels of RAID like RAID 10? It's becoming more popular nowadays so maybe you should also include it into your nice guide?

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Dr. Peaceful Aug 28, 2007, 02:45pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
I have a question. I am wondering what really happen (in block level) when you do defragmentation to a RAID array?

In a single hard drive setup, defragmentation will re-arrange file blocks, so blocks related to the same file will stay close together, hence improve accessing performance. Defragmenters also does compaction, so unused space can be more accessable.

Now in a RAID array, say RAID 0, for example. It's striped, with assigned strip size -- e.g. 16k, 32k, 64k, 128k, etc. So files, especially large ones, are split into 2 or more hard drives. Logically, in user's eyes, it appeared as a single drive. But really, when you do defragmentation, it occurs in multiple hard drives! Remember files are split across drives, how the hack it remembers where to find related file fragments in each drive after it's been re-arranged?

I am a bit confused. Somebody please explain.

CrAsHnBuRnXp Aug 28, 2007, 03:01pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Dr. Peaceful said:
I have a question. I am wondering what really happen (in block level) when you do defragmentation to a RAID array?

In a single hard drive setup, defragmentation will re-arrange file blocks, so blocks related to the same file will stay close together, hence improve accessing performance. Defragmenters also does compaction, so unused space can be more accessable.

Now in a RAID array, say RAID 0, for example. It's striped, with assigned strip size -- e.g. 16k, 32k, 64k, 128k, etc. So files, especially large ones, are split into 2 or more hard drives. Logically, in user's eyes, it appeared as a single drive. But really, when you do defragmentation, it occurs in multiple hard drives! Remember files are split across drives, how the hack it remembers where to find related file fragments in each drive after it's been re-arranged?

I am a bit confused. Somebody please explain.

The same probably occurs in defragmentation as it does when data is written to the hard drive. The blocks are sent to an identical location on both hard drives so that they can be retrieved for later use. If each block on the hard drive was moved to two (or more) separate locations during defragmentation, the data would get corrupt.

Dr. Peaceful Aug 28, 2007, 05:37pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Mark Allen said:
The same probably occurs in defragmentation as it does when data is written to the hard drive. The blocks are sent to an identical location on both hard drives so that they can be retried for later use. If each block on the hard drive was moved to two (or more) separate locations during defragmentation, the data would get corrupt.


You mean it's put in identical location (cylinder) in both hdd, even when the file blocks are moved during defrag?

CrAsHnBuRnXp Aug 28, 2007, 10:16pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Right. The defragger would move the file(s) to their new (same) location on the hard drives so that way they can be accessed when needed. Thats the way I would think it would work.

brian pope Nov 15, 2007, 11:08pm EST Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Regarding defraging RAID arrays-

Mirrored arrays- while the data on your mirrored arrays will be identical the location of the data on each drive won't be. This is true prior to defragmenting & afterwards, mirroring only means the data itself is mirrored not it's location on each hard drive, the MFT provides the OS with the location.

Stripped arrays- Obviously the data can be anywhere on each hard drive, a file can also be located on just one drive or spread out over numerous hard drives. When defragmenting RAID arrays the defrag software sees the RAID environment just as the file system does, meaning it defragments the virtual drive. Take a basic two-drive array, it would defragment the pieces of the file on each separate drive as though they were separate files & move each piece of that file as directed by the RAID controller. If piece 1 & 3 were on drive A & piece 2 & 4 were on drive B, your defrager would try to put pieces 1 & 3 together on drive A & the same with pieces 2 & 4 on drive B.

Your thinking they're already together when the file was created but that's not always the case. This takes you right back to why fragmentation occurs & why you should defrag your hard drives. The adding & removing of files creates spaces between files & the next newly created file will take it's place in the first available empty space.

Andrew McQuaide Feb 04, 2009, 07:13pm EST Reply - Quote - Report Abuse
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Edited: Feb 04, 2009, 07:14pm EST

 
>> Re: RAID Arrays Explained
I have two 250GB 7200rpm drives, which of these would be my fastest setup?

RAID0 - It would 'combine' my drives into 500GB correct? Fastest setup but high risk..?
RAID1 - My C:/ would be mirrored onto my D:/..does this mean longer write times? Does it mean read times are quicker?

Or, i could run my computer off my first drive, and keep all media (music, videos, pics, bulk, on my other drive and it would act in the same way an external would.

Thanks

Gerritt Feb 08, 2009, 03:08pm EST Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Andrew,
You are confusing the physical drives with the logical RAID, then the high level format.
Any RAID level with any number of physical HDDs, with a single established partition would be visable as a single drive, or C:\ with a size of 500GB, in your case of a RAID 0, or 250GB in the case of a RAID 1 (this is the usual configuration for low end RAID implementations).
However, you can create multiple virtual partitions within a single RAID Array just as you can on a single HDD, thus you could have a 500GB Raid Array with a C:\ partition with a size of 100GB and a D:\ partition of 400GB, or any other segmentation that is permissible by the controlling OS. Thus you can have Linux, Microsoft, Apple, etc, partitions defined within a single array, but they would not be visible to each other (some exceptions do apply though).

In your case of keeping the drives separate, this is called JBOD or Just a Bunch of Drives. You would get no redundancy, or speed increases, but it is the simplest configuration.

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angryhippy Jun 29, 2009, 02:28pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Is there any problems with using different brand drives? I have 2 300gig drives a Seagate and a WD. And do larger drives affect speed? Like 80gig vs. 300gig.

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CrAsHnBuRnXp Jun 29, 2009, 05:13pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
angryhippy said:
Is there any problems with using different brand drives? I have 2 300gig drives a Seagate and a WD. And do larger drives affect speed? Like 80gig vs. 300gig.

I dont see any issues with using a 300GB Seagate and a 300GB WD as they should show the same amount of space because of the way windows formats hard drives.

As for speed with 80 vs 300, I would say that the 80GB model would have a quicker access time than a 300GB.

Gerritt Jun 30, 2009, 11:25pm EDT Reply - Quote - Report Abuse
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>> Re: RAID Arrays Explained
Your whole array will function at the lowest speed of the drives that you have as members.

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