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RAID Disc System

Another important issue for discussion in determining the specifications - server redundancy. There are several methods of improving the reliability of disk systems from multiple disks. Most of these schemes redundancy - RAID variation (stands for, or redundant array of inexpensive disks independent;). The original specification RAID has been designed to replace the large and expensive disks mainframe minicomputers and arrays of small and cheap discs designed for minicomputers, hence the word inexpensive. "Unfortunately, the systems RAID rare something inexpensive. RAID implementations is a series of redundant disk arrays for provide different levels of protection and the speed of data transmission. As involves the use of RAID disk arrays, the best the application will interface to SCSI, because it can support up to 15 devices. RAID Levels there 6: from zero to fifth. While some manufacturers advertise their own scheme redundancy, which they called RAID-6, RAID-7 or above. (RAID-2 and RAID-4 is not on the network servers, so we are not going to talk.) Of all levels of RAID zero is the highest productivity and lowest security. It implies the existence of at least two devices, and synchronized recording data on both disks, the disks look like one physical device. The process of recording data at multiple disks called filling disks (drive spanning), a proper method of the data-recording their alternating (data striping). When sequencing data are written at all poblochno disk, a process called delamination blocks (block interleaving). The block size is determined by the operating system, but it usually ranging from 2 KB to 64 KB. Depending on the design of disk controller and HBA, the sequential operation may overlap, resulting in productivity increases. Thus, in itself RAID-0 can increase productivity, but does not provide protection from the bug. If a crash occurs, so goes the entire subsystem down, which usually leads to a complete loss of data. option is alternating data distribution data (data scattering). As with penetration, data are recorded consistently for a few discs filled. But unlike the sandwich record is not necessarily done on all disks, or if the drive is busy, data can be recorded on the next drive is accessible lets you add drives to existing vols. As standard RAID-0, the combination fill discs with the distribution of data improves productivity and increases the volume, but does not provide protection against failures RAID-1, also known as mirroring disk (disk mirroring), involves the installation of pairs of identical drives, each drive in a pair is a mirror of another. In RAID-1 data are written into two identical or nearly identical pairs of disks: when, for example, a disc degenerates, the system continues to work with a mirror disk. If the mirror disks are common HBA, the performance of this configuration, compared with odnodiskovoy, will be less because the data should be recorded in sequence on each disc.
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