The major benefit of RAID 5 is that it offers resilience to failure with minimum RAID overhead, maximising return on investment.
The minimum number of disks in a RAID 5 set is three (two for data and one for parity). The maximum number of drives in a RAID 5 set is in theory unlimited, although your storage array is likely to have built-in limits. However, RAID 5 only protects against a single drive failure. Two failures within a RAID 5 set will result in data corruption. Increasing the number of drives in your RAID 5 set increases your return on investment but it also increases the likelihood of simultaneous drive failures and lengthens RAID rebuild times. You must consider this trade-off when deciding how many drives to use in a RAID 5 set.
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Seven drives in a 6+1 RAID 5 configuration is a standard configuration in many storage environments. Hot spares can be used to mitigate some risk following a drive failure, although the data remains unprotected while rebuilding occurs. If the risk is deemed unacceptable, you should consider RAID 10 or RAID 6 as alternatives, although they will be more expensive to deploy on a "£/usable GB" basis.
More on RAID 5
Using RAID 1 and RAID 5 in a virtual server environment
How to calculate available disk space on a RAID 5 group
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