RAID Levels, continued

• RAID-1 (disk mirror): two or more disks of the same size store an exact copy of all data.
• Disks should be of the same size
• Best redundancy, data remains intact if at least one disk is good
• Least efficient use of storage, maximum overhead
• Good read performance for multitasking system (read requests are served from all disks at once)
• Write performance limited by the slowest disk (write is completed when all disks store their copy)
• Any one disk can be mounted as a (non-redundant) filesystem with all data intact
• RAID-4: two or more disks are combined through striping, one more disk contains parity data (XOR)
• Disks should be of the same size
• Array will recover if one disk is lost, but not two or more
• Minimal storage overhead (smaller for large arrays)
• Read performance is the same as for level 0
• All writes involve parity disk, write performance is degraded
• Rarely used, mostly superseded by RAID-5