<Ul> <Li> Array space efficiency is given as an expression in terms of the number of drives, n; this expression designates a fractional value between zero and one, representing the fraction of the sum of the drives' capacities that is available for use . For example, if three drives are arranged in RAID 3, this gives an array space efficiency of 1 − 1 / n = 1 − 1 / 3 = 2 / 3 ≈ 67%; thus, if each drive in this example has a capacity of 250 GB, then the array has a total capacity of 750 GB but the capacity that is usable for data storage is only 500 GB . </Li> </Ul> <Li> Array space efficiency is given as an expression in terms of the number of drives, n; this expression designates a fractional value between zero and one, representing the fraction of the sum of the drives' capacities that is available for use . For example, if three drives are arranged in RAID 3, this gives an array space efficiency of 1 − 1 / n = 1 − 1 / 3 = 2 / 3 ≈ 67%; thus, if each drive in this example has a capacity of 250 GB, then the array has a total capacity of 750 GB but the capacity that is usable for data storage is only 500 GB . </Li> <Table> <Tr> <Th> Level </Th> <Th> Description </Th> <Th> Minimum number of drives </Th> <Th> Space efficiency </Th> <Th> Fault tolerance </Th> <Th> Read performance </Th> <Th> Write performance </Th> </Tr> <Tr> <Th_colspan="2"> as factor of single disk </Th> </Tr> <Tr> <Td> RAID 0 </Td> <Td> Block - level striping without parity or mirroring </Td> <Td> </Td> <Td> </Td> <Td> None </Td> <Td> n </Td> <Td> n </Td> </Tr> <Tr> <Td> RAID 1 </Td> <Td> Mirroring without parity or striping </Td> <Td> </Td> <Td> 1 / n </Td> <Td> n − 1 drive failures </Td> <Td> n </Td> <Td> </Td> </Tr> <Tr> <Td> RAID 2 </Td> <Td> Bit - level striping with Hamming code for error correction </Td> <Td> </Td> <Td> 1 − 1 / n log (n + 1) </Td> <Td> One drive failure </Td> <Td> Depends </Td> <Td> Depends </Td> </Tr> <Tr> <Td> RAID 3 </Td> <Td> Byte - level striping with dedicated parity </Td> <Td> </Td> <Td> 1 − 1 / n </Td> <Td> One drive failure </Td> <Td> (n − 1) </Td> <Td> (n − 1) </Td> </Tr> <Tr> <Td> RAID 4 </Td> <Td> Block - level striping with dedicated parity </Td> <Td> </Td> <Td> 1 − 1 / n </Td> <Td> One drive failure </Td> <Td> 1 − (1 − r) − nr (1 − r) </Td> <Td> (n − 1) </Td> </Tr> <Tr> <Td> RAID 5 </Td> <Td> Block - level striping with distributed parity </Td> <Td> </Td> <Td> 1 − 1 / n </Td> <Td> One drive failure </Td> <Td> n </Td> <Td> single sector: 1 / 4 full stripe: (n − 1) </Td> </Tr> <Tr> <Td> RAID 6 </Td> <Td> Block - level striping with double distributed parity </Td> <Td> </Td> <Td> 1 − 2 / n </Td> <Td> Two drive failures </Td> <Td> n </Td> <Td> single sector: 1 / 6 full stripe: (n − 2) </Td> </Tr> </Table> <Tr> <Th> Level </Th> <Th> Description </Th> <Th> Minimum number of drives </Th> <Th> Space efficiency </Th> <Th> Fault tolerance </Th> <Th> Read performance </Th> <Th> Write performance </Th> </Tr>

What is the minimum number of drives needed to implement raid 6
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