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General Info

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RAID is a method where two or more disks are used to store data. Data can be read simultaneously from more than one drive providing better performance. The can provide redundancy to the data by splitting it among drives in bits, byte or blocks and by providing for duplication of the data to an extra drive. There are seven levels of RAID available 0,1,2,3,4,5 and 10, as shown below.

Basically 2 or more disks are connected together. A single controller can be used for all drives. For extra safety, a second interface controller can be installed providing to duplex the drives and increase read performance. Data striping and parity drives are also used to provide redundancy and better performance.

RAID 0 - Data stripping and Block Interleave ( minimum 2 drives needed )

Data is written to each drive in succession, each block going to the next available drive thus the data is distributed across the array drives (striping) providing faster operation and less chance that one drive will get overloaded with data requests.. The volume can of course be much larger than any single drive. Since no redundancy is provided, the failure of a single drive will bring the system down.

RAID 1 - Disk Mirroring and Duplexing ( minimum 2 drives needed )

Drives are used in pairs and all data is written identically to both drives. Each drive can be duplexed by being connected to its own interface controller. The failure of one drive will not bring down the system instead the other drive will continue to operate. Of course two drives are now used for the equivalent storage capacity of one drive.
There is no performance gain with this level.

RAID 2 - Data striping and bit interleave

Data is written across each drive in succession one bit at a time. Checksum data is recorded in a separate drive. This method is very slow for disk writes..

RAID 3 - Data striping with bit interleave and parity checking

This level is similar to lever 2 but more reliable. Data striping is done across the drives, one byte at a time. Usually 4 or 5 drives are used providing very high data transfer rates. One drive is dedicated to storing parity information. The failure of a single drive can be compensated by using the parity drive to reconstruct the failed drive contents. Since the parity drive is accessed on every write operation the writing of data tends to be slower. The failure of two drives or more can be a problem.

RAID 3 is not very good for Novell NetWare 3.x due to the use of 4K block sizes (too short)

RAID 4 - Block interleave data striping with parity checking

As in level 3, RAID 4 uses a single parity drive and block data striping like in RAID 0. The drives in this RAID level function individually, with an individual drive reading a block of data. A failure of the controller will of course be catastrophic.

RAID 5 - Block interleave, data striping with distributed check-data on all drives

The one to use for Netware.

Parity information is distributed across all drives. RAID 5 efficiency goes up as the number of disks increases. You can use hot spares to rebuild a failed drive on "the fly".

RAID 10 -

Much like NetWare's SFT III data is mirrored across two identical RAID 0 array drives

Links

• RAID Info @ AC&NC - has good descriptions for all RAID levels

Notes:

• This service is provided in NT 4.0 Server as a software implementation

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