Mirrored disks provide a shared location for cluster-aware applications to save data. Mirrored disks (applicable to NEC clusters only) are separate disk devices that are physically attached to their host separately, but work like one single device logically. Mirrored disks contain an exact duplicate of the disk that it mirrors. Data is stored twice by writing to both the local disk and its remote mirrored disk. If a disk fails, data does not have to be rebuilt and can be easily recovered by copying it from the mirrored disk to the replacement disk. It is recommended that mirrored disks reside on different devices so that a single-point disk failure cannot damage both copies of the data. The main disadvantage of mirrored disks is that the effective storage capacity is only half of the total disk capacity because all data gets written twice. The cluster system is configured so that only the active node can access the mirrored volume and sync-up the data between different physical disks.
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