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RAID 5 recovery: What is the maximum number of physical drives in a RAID 5 configuration?

Adding drives to a RAID 5 set increases your ROI and the likelihood of simultaneous drive failures, and lengthens RAID rebuild times. So what should be your physical drive limit?

What is the maximum number of physical drives in a RAID 5 configuration? For example, will data recovery in case of drive failure(s) be practical if I have seven 146 GB drives?
RAID 5 recovery: A RAID 5 set stripes data and parity information across multiple drives. If a drive fails, data is rebuilt from parity information.

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.

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

This was first published in August 2010

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