KNOW MORE ABOUT STORAGE VIRTUALIZATION USING RAID

KNOW MORE ABOUT STORAGE VIRTUALIZATION USING RAID

A disk subsystem with a RAID controller offers greater functional scope than a JBODdisk subsystem. RAID was originally developed at a time when hard disks were still veryexpensive and less reliable than they are today. RAID was originally called 'RedundantArray of Inexpensive Disks'. Today RAID stands for 'Redundant Array of IndependentDisks'. Disk subsystems that support RAID are sometimes also called RAID arrays.RAID has two main goals: to increase performance by striping and to increase fault-tolerance by redundancy. Striping distributes the data over several hard disks and thus distributes the load over more hardware. Redundancy means that additional information is stored so that the operation of the application itself can continue in the event of the failure of a hard disk. You cannot increase the performance of an individual hard disk any more than you can improve its fault-tolerance. Individual physical hard disks are slow and have a limited life-cycle. However, through a suitable combination of physical hard disks it is possible to significantly increase the fault-tolerance and performance of the system as a whole.The bundle of physical hard disks brought together by the RAID controller are also known as virtual hard disks. A server that is connected to a RAID system sees only the virtual hard disk; the fact that the RAID controller actually distributes the data overseveral physical hard disks is completely hidden to the server  This is only visible to the administrator from outside.A RAID controller can distribute the data that a server writes to the virtual hard disk amongst the individual physical hard disks in various manners. These different procedures are known as RAID levels. Section 2.5 explains various RAID levels in detail.One factor common to almost all RAID levels is that they store redundant information.If a physical hard disk fails, its data can be reconstructed from the hard disks that remain intact. The defective hard disk can even be replaced by a new one during operation if adisk subsystem has the appropriate hardware. Then the RAID controller reconstructs the data of the exchanged hard disk. This process remains hidden to the server apart from a possible reduction in performance: the server can continue to work uninterrupted on the

virtual hard disk.Modern RAID controllers initiate this process automatically. This requires the definitionof so-called hot spare disks  The hot spare disks are not used in normaloperation. If a disk fails, the RAID controller immediately begins to copy the data of theremaining intact disk onto a hot spare disk. After the replacement of the defective disk, this is cluded in the pool of hot spare disks. Modern RAID controllers can manage a common pool of hot spare disks for several virtual RAID disks. Hot spare disks can be defined for all RAID levels that offer redundancy.The recreation of the data from a defective hard disk takes place at the same time as write and read operations of the server to the virtual hard disk, so that from the oint ofview of the server, performance reductions at least can be observed. Modern hard disks come with self-diagnosis programs that report an increase in write and read errors to the system administrator in plenty of time: 'Caution! I am about to depart this life. Please replace me with a new disk. Thank you!' To this end, the individual hard disks store the data with a redundant code such as the Hamming code. The Hamming code permits the correct recreation of the data, even if individual bits are changed on the hard disk. If the system is looked after properly you can assume that the installed physical hard disks will hold out for a while. Therefore, for the benefit of higher performance, it is generally an acceptable risk to give access by the server a higher priority than the recreation of the data of an exchanged physical hard disk.A further side-effect of the bringing together of several physical hard disks to form a virtual hard disk is the higher capacity of the virtual hard disks. As a result, less device addresses are used up in the I/O channel and thus the administration of the server is also simplified, because less hard disks (drive letters or volumes) need to be used.