Is Raid 0 Worth It?

Skillax · Apr 1, 2006 at 10:04 AM

what is raid 0

XMasterX · Apr 1, 2006 at 11:27 AM

you ash... try en.wikipedia.org and look it up...

Equivalent Exchange · Apr 1, 2006 at 2:28 PM

Okay, since some people are too lazy to read wikipedia...

Raid is two hard drives working in unison instead of just one. It serves two purposes mainly, and those are to eaither increase performance or make sure you dont lose important data. Generally raid (Redundant array of independant disks) is reading/writing the same thing to 2 hard disks instead of one.

Raid 0 is for performance. Both disks will have the same info written to them. An example is best used for this setup.

In Raid 0, if one hard drive was busy writing and something else needed to be written, then the other hard drive would take over and write that data while the other drive finishes. It goes the same with reading data. If one is too busy, then the data will be read from the other. THe thing is, if one goes, then the other one does.

Raid 1 is making it so information is always written to two different drives, so if one fails, then you still have the other one. This is meant mainly for keeping the information on your computer safe from disk crashes.

Summary
Raid 0 is for performance
Raid 1 is for protecting information from disk failures

Hope that helps

Edit: If I'm wrong or anything, someone correct me, I'm just going off memory there

XMasterX · Apr 1, 2006 at 2:31 PM

well, Skillax is spamming alot of threads, so thanks anyway..

Originally posted by wikipedia
RAID 0

A RAID 0 (also known as a striped set) splits data evenly across two or more disks with no parity information for redundancy. It is important to note that RAID 0 was not one of the original RAID levels, and is not redundant. RAID 0 is normally used to increase performance, although it can also be used as a way to create a small number of large virtual disks out of a large number of small physical ones. A RAID 0 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 100 GB disk, the size of the array will be 200 GB. Although RAID 0 was not specified in the original RAID paper, an idealized implementation of RAID 0 would split I/O operations into equal-sized blocks and spread them evenly across two disks. RAID 0 implementations with more than two disks are also possible, however the reliability of a given RAID 0 set is equal to the average reliability of each disk divided by the number of disks in the set. That is, reliability (as measured by mean time to failure (MTTF) or mean time between failures (MTBF) is roughly inversely proportional to the number of members—so a set of two disks is roughly half as reliable as a single disk. The reason for this is that the file system is distributed across all disks. When a drive fails the file system cannot cope with such a large loss of data and coherency since the data is "striped" across all drives. Data can be recovered using special tools. However, it will be incomplete and most likely corrupt.

While the block size can technically be as small as a byte it is almost always a multiple of the hard disk sector size of 512 bytes. This lets each drive seek independently when randomly reading or writing data on the disk. If all the accessed sectors are entirely on one disk then the apparent seek time would be the same as a single disk. If the accessed sectors are spread evenly among the disks then the apparent seek time would be reduced by half for two disks, by two-thirds for three disks, etc., assuming identical disks. For normal data access patterns the apparent seek time of the array would be between these two extremes. The transfer speed of the array will be the transfer speed of all the disks added together.

RAID 0 is useful for setups such as large read-only NFS servers where mounting many disks is time-consuming or impossible and redundancy is irrelevant. Another use is where the number of disks is limited by the operating system. In Microsoft Windows, the number of drive letters for hard disk drives may be limited to 24, so RAID 0 is a popular way to use more disks. It is also a popular choice for gaming systems where performance is desired, data integrity is not very important, but cost is a consideration to most users. However, since data is shared between drives without redundancy, hard drives cannot be swapped out as all disks are dependent upon each other.
[edit]

RAID 1

A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful when read performance is more important than minimizing the storage capacity used for redundancy. The array can only be as big as the smallest member disk, however. A classic RAID 1 mirrored pair contains two disks, which increases reliability by a factor of two over a single disk, but it is possible to have many more than two copies. Since each member can be addressed independently if the other fails, reliability is a linear multiple of the number of members. To truly get the full redundancy benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing.

When reading, both disks can be accessed independently. Like RAID 0 the average seek time is reduced by half when randomly reading but because each disk has the exact same data the requested sectors can always be split evenly between the disks and the seek time remains low. The transfer rate would also be doubled. For three disks the seek time would be a third and the transfer rate would be tripled. The only limit is how many disks can be connected to the controller and its maximum transfer speed. Many older IDE RAID 1 cards read from one disk in the pair, so their read performance is that of a single disk. Some older RAID 1 implementations would also read both disks simultaneously and compare the data to catch errors. The error detection and correction on modern disks makes this less useful in environments requiring normal commercial availability. When writing, the array performs like a single disk as all mirrors must be written with the data.

RAID 1 has many administrative advantages. For instance, in some 365*24 environments, it is possible to "Split the Mirror": declare one disk as inactive, do a backup of that disk, and then "rebuild" the mirror. This requires that the application support recovery from the image of data on the disk at the point of the mirror split. This procedure is less critical in the presence of the "snapshot" feature of some filesystems, in which some space is reserved for changes, presenting a static point-in-time view of the filesystem. Alternatively, a set of disks can be kept in much the same way as traditional backup tapes are.

Also, one common practice is to create an extra mirror of a volume (also known as a Business Continuance Volume or BCV) which is meant to be split from the source RAID set and used independently. In some implementations, these extra mirrors can be split and then incrementally re-established, instead of requiring a complete RAID set rebuild.

Traditional
RAID 1
A1 A1
A2 A2
A3 A3
A4 A4

Note: A1, A2, et cetera each represent one data block; each column represents one disk.
[edit]

RAID 2

A RAID 2 stripes data at the bit (rather than block) level, and uses a Hamming code for error correction. The disks are synchronized by the controller to run in perfect tandem. This is the only original level of RAID that is not currently used. Extremely high data transfer rates are possible.
[edit]

RAID 3

A RAID 3 uses byte-level striping with a dedicated parity disk. RAID 3 is very rare in practice. One of the side effects of RAID 3 is that it generally cannot service multiple requests simultaneously. This comes about because any single block of data will by definition be spread across all members of the set and will reside in the same location, so any I/O operation requires activity on every disk.

In our example below, a request for block "A" consisting of bytes A1-A9, would require all three data disks to seek to the beginning(A1) and reply with their contents. A simultaneous request for block B would have to wait.

Traditional
RAID 3
A1 A2 A3 Ap(1-3)
A4 A5 A6 Ap(4-6)
A7 A8 A9 Ap(7-9)
B1 B2 B3 Bp(1-3)

Note: A1, B1, etcetera each represent one data byte; each column represents one disk.
[edit]

RAID 4

A RAID 4 uses block-level striping with a dedicated parity disk. RAID 4 looks similar to RAID 3 except that it stripes at the block, rather than the byte level. This allows each member of the set to act independently when only a single block is requested. If the disk controller allows it, a RAID 4 set can service multiple read requests simultaneously.

In our example below, a request for block "A1" would be serviced by disk 1. A simultaneous request for block B1 would have to wait, but a request for B2 could be serviced concurrently.

Traditional
RAID 4
A1 A2 A3 Ap
B1 B2 B3 Bp
C1 C2 C3 Cp
D1 D2 D3 Dp

Note: A1, B1, et cetera each represent one data block; each column represents one disk.
[edit]

RAID 5

A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 has achieved popularity due to its low cost of redundancy. Generally RAID 5 is implemented with hardware support for parity calculations.

In the example below, a read request for block "A1" would be serviced by disk 1. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently.

Traditional
RAID 5
A1 A2 A3 Ap
B1 B2 Bp B3
C1 Cp C2 C3
Dp D1 D2 D3

Note: A1, B1, et cetera each represent one data block; each column represents one disk.

Every time a block is written to a disk in a RAID 5, a parity block is generated within the same stripe. A block is often composed of many consecutive sectors on a disk. A series of blocks (a block from each of the disks in an array) is collectively called a "stripe". If another block, or some portion of a block, is written on that same stripe the parity block (or some portion of the parity block) is recalculated and rewritten. For small writes, this requires reading the old parity, reading the old data, writing the new parity, and writing the new data. The disk used for the parity block is staggered from one stripe to the next, hence the term "distributed parity blocks". RAID 5 writes are expensive in terms of disk operations and traffic between the disks and the controller.

The parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error. In this case, the sector in the same relative position within each of the remaining data blocks in the stripe and within the parity block in the stripe are used to reconstruct the errant sector. The CRC error is thus hidden from the main computer. Likewise, should a disk fail in the array, the parity blocks from the surviving disks are combined mathematically with the data blocks from the surviving disks to reconstruct the data on the failed drive "on the fly".

This is sometimes called Interim Data Recovery Mode. The computer knows that a disk drive has failed, but this is only so that the operating system can notify the administrator that a drive needs replacement; applications running on the computer are unaware of the failure. Reading and writing to the drive array continues seamlessly, though with some performance degradation. The difference between RAID 4 and RAID 5 is that, in interim data recovery mode, RAID 5 might be slightly faster than RAID 4, because, when the CRC and parity are in the disk that failed, the calculation does not have to be performed, while with RAID 4, if one of the data disks fails, the calculations have to be performed with each access.

In RAID 5, where there is a single parity block per stripe, the failure of a second drive results in total data loss.

The maximum number of drives in a RAID 5 redundancy group is theoretically unlimited, but it is common practice to limit the number of drives. The tradeoffs of larger redundancy groups are greater probability of a simultaneous double disk failure, the increased time to rebuild a redundancy group, and the greater probability of encountering an unrecoverable sector during RAID reconstruction. As the number of disks in a RAID 5 group increases, the MTBF can become lower than that of a single disk. This happens when the likelihood of a second disk failing out of (N-1) dependent disks, within the time it takes to detect, replace and recreate a first failed disk, becomes larger than the likelihood of a single disk failing. RAID 6 is an alternative that provides dual parity protection thus enabling larger numbers of disks per RAID group.

Some RAID vendors will avoid placing disks from the same manufacturing run in a redundancy group to minimize the odds of simultaneous early life and end of life failures as evidenced by the bathtub curve.

RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe; this is because parity must be updated on each write, requiring read-modify-write sequences for both the data block and the parity block. More complex implementations often include non-volatile write back cache to reduce the performance impact of incremental parity updates.

In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data; if this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe; this potential vulnerability is sometimes known as the "write hole". Battery-backed cache and other techniques are commonly used to reduce the window of vulnerability of this occurring.
[edit]

RAID 6

A RAID 6 extends RAID 5 by adding an additional parity block, thus it uses block-level striping with two parity blocks distributed across all member disks. It was not one of the original RAID levels.

RAID 5 can be seen as a special case of a Reed-Solomon code where the syndrome used is the one built from generator 1 [1]. Thus RAID 5 only requires addition in the galois field. Since we are operating on bytes, the field used is a binary galois field (), typically of order 8. In binary galois fields, addition is computed by a simple XOR.

After understanding RAID 5 as a special case of a Reed-Solomon code, it is easy to see that it is possible to extend the approach to produce redundancy simply by producing another syndrome using a different generator; for example, 2 in . By adding additional generators it is possible to achieve any number of redundant disks, and recover from the failure of that many drives anywhere in the array.

Like RAID 5 the parity is distributed in stripes, with the parity blocks in a different place in each stripe.

Traditional Typical
RAID 5 RAID 6
A1 A2 A3 Ap A1 A2 A3 Ap Aq
B1 B2 Bp B3 B1 B2 Bp Bq B3
C1 Cp C2 C3 C1 Cp Cq C2 C3
Dp D1 D2 D3 Dp Dq D1 D2 D3

Note: A1, B1, et cetera each represent one data block; each column represents one disk;
p and q represent the two Reed-Solomon syndromes.

RAID 6 is inefficient when used with a small number of drives but as arrays become bigger and have more drives the loss in storage capacity becomes less important and the probability of two disks failing at once becomes greater. RAID 6 provides protection against double disk failures and failures while a single disk is rebuilding. In the case where there is only one array it makes more sense than having a "hot spare" disk.

The user capacity of a RAID 6 array is n-2, where n is the total number of drives in the array.

RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations due to the overhead associated with the additional parity calculations.
Click to expand...

johndapunk FTW · Apr 1, 2006 at 2:44 PM

Originally posted by Skillax@Apr 1 2006, 02:04 PM
what is raid 0

[snapback]2320785[/snapback]
Click to expand...

ok, one, look at this image in DETAIL

two, admin, please like ban this spamming faggot?

and i guess ill add something on topic, there are more different kinds of RAID setup's then what XmasterX posted, just they arent popular.

XMasterX · Apr 1, 2006 at 2:49 PM

yes, there are many more, though only RAID 0-6 are used mostly. the others, yeah, some companies use them still, but its not very comon..

Blooper008 · Apr 2, 2006 at 10:05 AM

Its like 0 till 15...
Btw: Master, didnt you had the Master's sex slave button, or was it someone else ?

Sivartus · Apr 11, 2006 at 11:16 PM

Ok, back to if RAID 0 is worth it or not. Anyone have any sites showing the benchmarks of whether or not RAID 0 gives significant performance increase or not, for the money?

XMasterX · Apr 11, 2006 at 11:19 PM

read our posts, that should be all the info you need...

xlink · Apr 11, 2006 at 11:33 PM

http://www.anandtech.com/storage/showdoc.aspx?i=2101

as you see a single 74 GB raptor offers ABOUT a 50% performance boost over a standard harddrive, yet raiding two only deleivers around a 10-20% boos on average, it might be in part due to the lower cache f the 74Gb raptor, but typically, I've heard that a single WD1500 come close to or outperforms two hitachi diskstar drives(highest performance 7200rpm drives out there as far as i know) in raid.

might want to check this though:
http://www.storagereview.com/php/cms/cms.p...tart=6&range=10

unless you are transfering enormous files raid doesn't increase performance as phenominally as many beleive. it took me a year since finding this out to actually understand why. Raid doesn't improve the time which it takes a harddrive to find a file, if you are opening 1000 VERY small files at once(booting windows, opening photoshop with thousands of fonts etc.) RAID does nothing and might even increase seek times(though WD claims its TLer technology helps avoid that) I know a guy with two hitachi diskstars in RAID and performance was compareale to my single WD1500 raptor.

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