Whic is the better HDD configuration ?
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A RAID array distributes data across several physical disks which look to the operating system and the user like a single disk. Several different arrangements are possible. We assume here that all the disks are of the same capacity, as is usual.
Some arrays are "redundant" in a way that writes extra data derived from the original data across the array organized so that the failure of one (sometimes more) disks in the array will not result in loss of data; the bad disk is replaced by a new one, and the data on it reconstructed from the remaining data and the extra data. A redundant array obviously allows less data to be stored; a 2-disk RAID 1 array loses half of its capacity, and a RAID 5 array with several disks loses the capacity of one disk.
Other RAID arrays are arranged in a way that makes them faster to write to and read from than a single disk.
There are various combinations of these approaches giving different tradeoffs of protection against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.
RAID 0 distributes data across several disks in a way which gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails.
RAID 1 (mirrored disks) uses two (possibly more) disks which each store the same data, so that data is not lost so long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure or decrease the reliability of the remaining drives(second, third, etc).
RAID 5 combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk. The less common RAID 6 can recover from the loss of two disks.
RAID involves significant computation when reading and writing information. With true hardware RAID the controller does the work. In other cases the operating system or simpler and less expensive controllers require the host computer's processor to do the computing, which reduces the computer's performance on processor-intensive tasks (see "Software RAID" and "Fake RAID" below). Simpler RAID controllers may provide only levels 0 and 1, which require less processing.
RAID systems with redundancy continue working without interruption when one, or sometimes more, disks of the array fail, although they are vulnerable to further failures. When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally. Some systems have to be shut down when removing or adding a drive; others support hot swapping, allowing drives to be replaced without powering down. RAID with hot-swap drives is often used in high availability systems, where it is important that the system keeps running as much of the time as possible.
It is important to note that redundant RAID is not an alternative to backing up data. Data may become damaged or destroyed without harm to the drive(s) on which it is stored. For example, part of the data may be overwritten by a system malfunction; a file may be damaged or deleted by user error or malice and not noticed for days or weeks; and of course the entire array is at risk of catastrophes such as theft, flood, and fire.
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RAID principles
RAID combines two or more physical hard disks into a single logical unit by using either special hardware or software. Hardware solutions often are designed to present themselves to the attached system as a single hard drive, and the operating system is unaware of the technical workings. Software solutions are typically implemented in the operating system, and again would present the RAID drive as a single drive to applications.
There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID levels use one or more of these techniques, depending on the system requirements. The main aims of using RAID are to improve reliability, important for protecting information that is critical to a business, for example a database of customer orders; or where speed is important, for example a system that delivers video on demand TV programs to many viewers.
The configuration affects reliability and performance in different ways. The problem with using more disks is that it is more likely that one will go wrong, but by using error checking the total system can be made more reliable by being able to survive and repair the failure. Basic mirroring can speed up reading data as a system can read different data from both the disks, but it may be slow for writing if the configuration requires that both disks must confirm that the data is correctly written. Striping is often used for performance, where it allows sequences of data to be read from multiple disks at the same time. Error checking typically will slow the system down as data needs to be read from several places and compared. The design of RAID systems is therefore a compromise and understanding the requirements of a system is important. Modern disk arrays typically provide the facility to select the appropriate RAID configuration.
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Standard levels
Main article: Standard RAID levels
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary).
A brief summary of the most commonly used RAID levels. The SNIA Dictionary also contains definitions of the RAID levels that have been vetted by major storage industry players, and is referenced below as applicable. (RAID 2 is not included here as there are no commercial implementations of that level.)Level Description Minimum # of disks Image
RAID 0 Striped set without parity. Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drive, the data is broken into fragments. The number of fragments is dictated by the number of disks in the drive. The fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the array means higher bandwidth, but greater risk of data loss. SNIA definition. 2
RAID 1 Mirrored set without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning. SNIA definition. Using RAID 1 with a separate controller for each disk is sometimes called duplexing. 2
RAID 3 Striped set with dedicated parity. This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty. SNIA definition 3
RAID 4 Identical to RAID 3 but does block-level striping instead of byte-level striping. SNIA definition 3
RAID 5 Striped set with distributed parity. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. SNIA definition 3
RAID 6 Striped set with dual parity. Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems. This becomes increasingly important because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if one drive, but no more, fails before the rebuild is complete
Wikipedia is your friend
I'd never put 2x320gb hdd's in raid 0. At least not at this point in time. 640gb as a main drive is way too large coupled with the risk of losing data or even having to format for whatever reason. 2x80gb or 2x120gb for raid 0 and even highly affordable for a raid 0+1. I'd rather go with smaller drives for raid 0 and a larger drive for back up and you wont even have to seriously consider raid 0+1.muirplayerWell, what about putting in a third 320 and going RAID5? You end up with 640GB of high-performance, fault-tolerant storage (it's how I'm set up right now).
I can provide RAID5 in a nutshell. I'm familiar with it since I'm using it. Like in RAID0, RAID5 stripes, or distributes, the data across the discs in the array. Only this time, as well as actual usable data, scattered evenly across the discs are bits of what's called parity data. For the sake of simplicity, think of a parity block as a special block designed to help rebuild the contents of a single block should it turn up in error. This parity data is the fault tolerance in a RAID5, and the fact it's distributed across all of the discs at a n-1:1 actual data-to-parity data ratio means RAID5 is capable of recovering from the failure of any one drive. Should one drive fail, the parity data on the other two drives can be used to rebuild the data when the third drive is replaced.
Well, what about putting in a third 320 and going RAID5? You end up with 640GB of high-performance, fault-tolerant storage (it's how I'm set up right now).[QUOTE="muirplayer"]I'd never put 2x320gb hdd's in raid 0. At least not at this point in time. 640gb as a main drive is way too large coupled with the risk of losing data or even having to format for whatever reason. 2x80gb or 2x120gb for raid 0 and even highly affordable for a raid 0+1. I'd rather go with smaller drives for raid 0 and a larger drive for back up and you wont even have to seriously consider raid 0+1.HuusAsking
I can provide RAID5 in a nutshell. I'm familiar with it since I'm using it. Like in RAID0, RAID5 stripes, or distributes, the data across the discs in the array. Only this time, as well as actual usable data, scattered evenly across the discs are bits of what's called parity data. For the sake of simplicity, think of a parity block as a special block designed to help rebuild the contents of a single block should it turn up in error. This parity data is the fault tolerance in a RAID5, and the fact it's distributed across all of the discs at a n-1:1 actual data-to-parity data ratio means RAID5 is capable of recovering from the failure of any one drive. Should one drive fail, the parity data on the other two drives can be used to rebuild the data when the third drive is replaced.
No listen to
I'd never put 2x320gb hdd's in raid 0. At least not at this point in time. 640gb as a main drive is way too large coupled with the risk of losing data or even having to format for whatever reason. 2x80gb or 2x120gb for raid 0 and even highly affordable for a raid 0+1. I'd rather go with smaller drives for raid 0 and a larger drive for back up and you wont even have to seriously consider raid 0+1.muirplayer
Why I do it, I just have a backup , I reformat often too, raid5 is another good alternative, I have it on my server. Like the guy above me he is right, but raid5 does not offer the best performence although it is a good alternative.
As you've said, performance isn't as good as a RAID 0 (since the controller has to skip reading parity blocks), but it's not that far off. And write performance does take a hit (even with a write-back cache), but as I do multimedia work, I work with lots of big files and find use for the vast array of space. My last rig had multiple partitions, but I found myself juggling too much.Why I do it, I just have a backup , I reformat often too, raid5 is another good alternative, I have it on my server. Like the guy above me he is right, but raid5 does not offer the best performence although it is a good alternative.
Renegade_Fury
As for backup, I leave that to a 320GB USB hard drive containing the computer as I had it when all my desired apps were installed--and a recovery boot CD. It's currently packed up and awaiting a rainy day (or the day I feel a spring clean is in order). The data I'll deal with as the need requires (deleting them, packing them off to DVDs or USB drives, etc.).
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