When any drive under RAID fails it informs the same through LED indication (or ‘Beep’), as well it can provide information over LAN (if supported and configured). At the same time the RAID CPU sets the state of other disks to ‘0’ (virtually, meaning those are available with data) and the failed slot to ‘1’. The administrator is supposed to replace the failed disk with a new one. On ‘snooping’ the new disk, it assigns the logic ‘1’ for the new disk (doing an ‘XOR’ function of all logical state of all other disks). Then the RAID logic starts ‘Rebuilding’ the data to the drive having logical state ‘1’ getting data from all logical state ‘0’ drives.
This kind of implementation is called ‘Hardware XOR’.
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