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Absolute FreeBSD, 2nd Edition by Michael W. Lucas

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Mirroring Disks

FreeBSD configures and manages mirrored disks with gmirror(8). While mirrors are traditionally pairs of disks, you can have a mirror with any number of disks. Mirrored disks are one of the simplest forms of fault tolerance; if a single drive fails, the data remains on the mirror. When choosing drives to mirror, it's best to make them physically separate: on different controllers, different cables, even different servers if possible. For this example, I'm using two SCSI hard drives, /dev/da0 and /dev/da1.

Mirroring a disk writes metadata at the very end of each partition, overwriting any data already there. I recommend only mirroring drives and partitions that are newly created, as any data on these partitions tends to be written ...

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