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... 1 is the traditional scheme for tolerating disk failure, also called mirroring or shadowing (see figure 2.2). This RAID approach level 1 defines uses twice the as user many data disks to be as RAID block-striped 0 because across whenever the mirror data are pairs. written Traditional to one disk, mirrored those systems data are rather also written fill each to a disk redundant with consecutive disk, thus there user data are always before two switching copies of to the the information next disk, leading which to can increased be thought reliability of as by setting a factor the stripe of two. unit Since to the reliability size of one is the disk. linear multiple of the number of member disks, it can easily be increased by creating more than two copies of data. RAID level 1 defines the user data to be block-striped across the mirror pairs. Traditional mirrored systems rather fill each disk with consecutive user data before switching to the next disk, which can be thought of as setting the stripe unit to the size of one disk. In a RAID level 3 setup, data is conceptually interleaved bit-wise over the data disks, with an additional parity disk tolerating a single disk failure (see figure 2.3). With this setup each read request needs to access all data disks and each write request needs to access all data disks and the parity disk. Thus it is only possible to serve one read or write request at a time. Having In If an a RAID error a complete occurs level 3 during setup, copy of a data the read original is operation, conceptually data the for corresponding interleaved each disk is bit-wise a very disk expensive controller over the solution. data reports disks, a read The with data cost an additional of error higher so the availability parity RAID disk system tolerating can be knows decreased a single which disk by disk only failure has adding failed, (see enough figure 2.3). redundant With this information, setup each referred read request to as needs parity , to to access restore all the data lost disks information and each write on a failure. request needs to access all data disks and the parity disk. Thus it is only possible to serve one read or write request at a time. If an error occurs during a read operation, the corresponding disk controller reports a read data error so the RAID system knows which disk has ...
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