HP Xw9300 Software RAID in Linux Workstations - Page 6

RAID-2: Error Checking and Correction RAID-2 adds error checking and correcting (ECC) checksums to RAID-1. ECC stands for either "Error Correcting Code" or "Error Checking and Correcting." It is a code in which each data signal conforms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected. RAID-2 is rarely seen these days because most hard-disk controllers already do ECC, and this scheme offers few advantages over other RAID configurations. Software RAID-2 is not supported by HP Linux workstations. RAID-3: Byte-Level Striping with Parity Disk Like RAID-0, RAID-3 does striping, but at a very small granularity. It also adds a parity disk which helps in error detection and recovery. Parity in regards to hard disks refers to use of a parity bit, which counts whether the number of 1 bits in some preceding data was even or odd; if a single bit is changed in transmission, the parity bit will change, thus providing a redundancy check for data transmission. Parity bits are a very simple example of ECCs, or Error Correcting Codes. The details of ECC and parity are beyond the scope of this paper. The small granularity of RAID-3 leads to gains only when record sizes are very small and drive spindles are carefully synchronized. RAID-5 is generally preferred, and RAID-3 is very seldom used these days. Software RAID-3 is not supported by HP Linux workstations. RAID-4: Block-Level Striping with Parity Disk RAID-4 attempts to add error checking and recovery to RAID-3 by doing block-level striping, as in RAID-0, with the addition of a single parity disk. At least 3 disks are required for a RAID-4 array. Since all operations access the parity disk, that disk can become a bottleneck. RAID-4 can work in some operations, but RAID-5 is generally preferred where a compromise between speed and reliability is sought. Software RAID-4 is not supported by HP Linux workstations. RAID-5: Block-Level Striping with Distributed Parity In order to add error checking and recovery to RAID-0 and to eliminate the parity disk bottleneck of RAID-4, RAID-5 is implemented as a combination of data striping and parity, where data and parity blocks are successively written across the drives of the array. RAID-5 is used to ensure data integrity; should a single disk fail, it is possible to recover the data on the lost disk from the parity data on the others. RAID-5 requires a minimum of 3 disks, and the effective disk space availability of n disks in a RAID-5 array is n-1 disks. Software RAID-5 is supported by HP Linux workstations. Figure 3. Efficiency of Software RAID-5 Read Performance Write Performance Space Efficiency Reliability Poor to Moderate Moderate 66-75% (3 and 4disk) Good For both small and large While RAID-5 eliminates RAID-5's efficiency is Parity data is retained blocks, RAID-5 performs the parity-disk bottleneck about the same as RAID- for all data. RAID-5 poorly because parity of RAID-4, block-write 4, where one disk is arrays can tolerate the data is interspersed with performance is still used to hold parity data, failure of one disk. real data. The slower than raw disks. except the parity data is performance of RAID-5 spread rather than can be dramatically concentrated on one improved by filesystem drive. tuning. 6