RAID
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RAID
RAID (Redundant Array of Inexpensive Disks) is a storage technology
that groups multiple hard drives into what appears to be one logical volume.
The term RAID was introduced in a late-1987 paper by Patterson, Gibson,
and Katz of the University of California-Berkley entitled "A Case for Redundant
Arrays of Inexpensive Disks (RAID)." The paper compared RAID to SLED (Single
Large Expensive Disk) and described five-disk array architectures, or levels.
RAID technology is currently the hottest mass storage topic in the literature.
Disk arrays generally improve system performance by supporting multiple
simultaneous read and/or write operations as well as by increasing capacity
and providing fault tolerance. The use of multiple drives in an array actually
increases the chances that a drive failure will occur. However, the data
redundancy of RAID allows the array to tolerate a drive failure. A basic
description of each of the levels of RAID follows.
RAID 0
This form of RAID is not RAID as described in the Berkley paper because
there is no data redundancy. Most disk arrays use striping, or distribution
of data across multiple drives. RAID 0 implements striping without redundancy
and is, therefore, less reliable than a single drive. The only advantage
is increased speed.
RAID 1
RAID 1 implements "mirroring," or shadowing, of disks. Each drive in the system has a copy, or "mirror," of itself. If a drive fails, the duplicate drive keeps working with no lost data or downtime.
Since there are two sources of data, the average access time for a read request will be faster than that for a single drive. For a write request, which is almost always preceded by a read, the decrease in read seek time of RAID 1 is offset by the increase in write seek time (since the data has to be written to two disks). A read and two writes takes the same time as a read/write for a single drive. RAID 1, with an optimized controller, has slightly lower overall access times than a single drive.
The main advantage of RAID 1 over other RAID architectures is simplicity. It only requires a dual channel controller or a minimal device driver using one or two controllers to implement. No change to the operating system is needed. RAID 1 is relatively expensive to implement because only half the available disk space is used for data storage. In addition, the necessity of duplicate drives requires more power and more space for the same storage capacity.
RAID 0/1 (sometimes also called 10) is a hybrid of RAID 0 and RAID 1.
The data is striped across the drives in the array as in RAID 0. In addition,
each striped drive group is "mirrored" by a duplicate drive group attached
to a second drive controller.
RAID 2
RAID 2 is an architecture that succeeds in reducing disk overhead (the cost of storage space lost to redundancy) by using Hamming codes to detect and correct errors. Check disks are required in addition to the data disks. The data is striped across the disks along with an interleaved Hamming code. Because all of the data disks must seek before a read starts, and because for a write the data disks must seek, read data, all drives (including check disks) must seek again, and then the data is written, seek times are very slow compared to a single drive. However, once the seek is completed, data transfer rates are very high. For an array with 8 data drives, the drives will transmit data in parallel. The transfer rate of the array will be 8 times that of a single drive.
RAID 2 is best for reading and writing large data blocks at high data
transfer rates. In the microcomputer environment the existing error detection/
correction features result in redundant error isolation data for RAID 2
and make RAID 2 impractical for microcomputers. By letting the drives manage
error detection, it is possible to implement RAID requiring only one check
disk for error correction.
RAID 3
By assuming that each disk drive in the array can detect and report errors, the RAID system only has to maintain redundancy in the data necessary to correct errors. RAID 3 employs a single check disk (parity disk) for each group of drives. Data is striped across each of the data disks. The check disk receives the XOR (exclusive OR) of all the data written to the data drives. Data for a failed drive can be reconstructed by computing the XOR of all the remaining drives. This approach reduces disk overhead from RAIDs 1 and 2. For a five-disk array, four of the drives store data; providing 4 GB of data storage in a 5 GB array. RAID 3 also has the same high transfer rates as RAID 2. However, because every data drive is involved in every read or write, a penalty is paid.
RAID 3 can process only one I/O transaction at a time. In addition,
the minimum amount of data that can be written or read from a RAID 3 array
is the number of data drives multiplied by the number of bytes per sector,
referred to as a transfer unit. A typical five-drive array would have four
data disks, one parity disk, and might have a 512-byte sector size on each
disk. The transfer unit would be 2048 bytes (4 x 512). When a data read
is smaller than the transfer unit, the entire unit is read anyway, increasing
the length of a read operation. For a data write smaller than the transfer
unit, although only a portion of a sector of each disk needs to be modified,
the array must still deal with complete transfer units. A complete unit
must be read from the array; the data must be rewritten where necessary;
and the modified data must be written back to the data disks and the check
disk updated. RAID 3 works well in applications that process large chunks
of data.
RAID 4
RAID 4 addresses the problems associated with bit-striping a transfer
block of data across the array. As in RAID 3, one drive in the array is
reserved as the check disk. This architecture, however, utilizes block
or sector striping to the drives, resulting in read transactions involving
only one drive and timing comparable to a single drive. In addition, multiple
read requests can be handled at the same time. However, since every write
accesses the parity disk, only one write at a time is possible. RAID 4
is most useful in an environment where the ratio of reads to writes is
very high.
RAID 5
Because RAID levels 2 through 4 each use a dedicated check disk, only one write transaction is possible at any time. RAID 5 overcomes the write bottleneck by distributing the error correcting codes (ECC) across each of the disks in the array. Therefore, each disk in the array contains both data and check-data.
Distributing check-data across the array allows reads and writes to
be done in parallel. Data recovery and seek times are comparable to RAID
4.
Disk Array Implementations
Actual disk array implementations are not always as simple or straightforward
as described above. Some manufacturers combine features of different RAID
levels to create a hybrid, as in RAID 0/1. RAID implementations that are
extremely fault-tolerant provide redundancy beyond that of the drives.
Additional redundancy is accomplished by providing a system with redundant
drive controllers, redundant power supplies, redundant SCSI controllers,
and so on. Some manufacturers offer "hot swappability," the ability to
replace a failed drive (or other hardware units) without shutting the system
down. Other manufacturers offer a spare drive that is automatically put
into use rebuilding the failed drive as soon as the system senses a failure.
Still other RAID manufacturers offer only software-based RAID. The RAID architecture is contained in software that the customer implements with his own hardware. And finally, some RAID systems offer more than one level of RAID in the same package to handle mixed applications more efficiently.