R.A.I.D. Configurations Types of RAID
RAID
This paper is going to provide a brief description of RAID and discuss various aspects of RAID. This will include the various levels of RAID, Hardware RAID, Software RAID, and the advantages and disadvantages associated with utilizing RAID.
RAID is an acronym for Redundant Array of Independent Disks. The basic idea behind RAID is to combine multiple small, inexpensive disk drives into an array to accomplish performance or redundancy goals not attainable with one large and expensive drive. The underlying concept of RAID is that data may be distributed across each drive in the array in a consistent manner. To do this, the data must first be broken into consistently sized chunks, often 32K or 64K in size, although different sizes can be used. Each chunk is then written to a hard drive in RAID according to the RAID level used. When the data is to be read, the process is reversed, giving the illusion that multiple drives are actually one large drive.
The main benefits in which RAID will provide are data protection and availability. RAID protects your data in the unlikely event of a drive failure. If a disk drive fails in a RAID system, network clients are unaware of the incident and they continue on with their work as if nothing happened. The RAID system continues to perform read/write operations and if a hot spare is available then it automatically becomes part of the array and data that was on the failed drive is automatically regenerated onto this new drive in the array. The various levels of RAID architectures as described by Charles M. Kozierok are defined below.
RAID Level 0 is a misnomer because the storage is not redundant, but it is an array. In RAID-0, data is interleaved across drives for higher data throughput. Because it stores no redundant information, its performance is very good, however the failure of any drive in the array results in data loss. RAID-0 is also known as striping. RAID-0 is the fastest and most efficient array type but offers no fault-tolerance.
RAID Level 1 provides redundancy by writing all data to two or more drives. RAID-1 tends to perform faster on reads and slower on writes compared to a single drive. There is no loss of data if either drive fails. RAID-1 is also known as mirroring. RAID-1 is a good choice for performance-critical, fault-tolerant environments. In addition, RAID-1 is the only choice for fault-tolerance if no more than two drives are desired.
RAID Level 2 is intended for use with drives that do not have built-in error correction by implementing error correction on the RAID controller. Since all SCSI drives support built-in error detection, RAID-2 adds unnecessary overhead when using SCSI drives.
RAID Level 3 stripes byte level data across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. Like RAID-0, the read performance of RAID-3 is very good for reads. Performance for small random writes suffers because parity data must be updated each time. Large writes or sequential writes are fairly fast. Because only one drive in the array stores redundant data, the cost per megabyte of RAID-3 can be fairly low. RAID-3 is a good choice for data intensive or single-user environments that access long sequential records to speed up data transfer.
RAID Level 4 stripes byte level data across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. Like RAID-0, the read performance of RAID-4 is very good for reads. Performance for small random writes suffers parity data must be updated each time. Large writes or sequential writes are fairly fast. Because only one drive in the array stores redundant data, the cost per megabyte of RAID-4 can be fairly low. RAID-4 offers no advantages over RAID-5 and does not support multiple simultaneous write operations.
RAID Level 5 is similar to RAID-4, but stores parity across the drives. Small writes in multiprocessing systems are faster because the parity disk is not a bottleneck. On reads, parity data must be skipped on each drive, so the read performance tends to be lower than a level 4 array. The cost per megabyte is the same as for RAID-4.
With a Hardware based RAID system, all RAID functions are handled independent of the host. The host sees only 1 disk per RAID set. They are very efficient because they do not occupy host system memory, they don’t consume CPU cycles and they are operating system independent. Hardware RAID is also highly fault tolerant because the array logic is based in hardware and software is not required to Boot.
Software based RAID systems offer a lower cost but they utilize system resources. They occupy host memory, consume CPU cycles and are operating system dependent. Software is also required for the array to boot and most implementations require a separate boot drive not included in the array.
To obtain a quantitative picture for determining the superiority of either hardware or software RAID, ADEPTEC Incorporated conducted a test. The test utilized the results obtained from using NetBench Disk test (version 7.0). NetBench is an application that measures the performance of file servers handling network file requests. The results in the table below show the superiority in performance of hardware RAID to software RAID. In conclusion hardware RAID is a superior solution to software RAID in a networked environment.
