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RAID 5 configuration: How to calculate disk space for data storage and parity storage

How do you calculate the disk space available for data storage and how much disk space is utilised for parity storage in a RAID 5 configuration?

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RAID 5, if applied correctly and against a suitable I/O profile, will provide one of the most cost-effective forms of protection available while affording the volumes it is servicing I/O performance equivalent to required needs.

Through distributing an addressable parity bit across each of the physical disks within the RAID group, the group can recover from the failure of a single disk by reading the parity storage data from each of the remaining disks and applying an algorithm against it to rebuild to a replacement disk.

So, how much overhead do you incur in the implementation of RAID 5 configuration? Simply put, when sizing a RAID 5 group, allow for the total amount of disks you need to suit your capacity needs + 1.

An example is given below:

Total required usable capacity = 750Gb
Size of drives available = 300Gb
Number of drives required for RAID 5 group = 4x300Gb drives (3+1)
Total usable capacity = 900Gb

Note: The total usable disk capacity of a given drive is very rarely equivalent to the total marketed capacity of that drive, due to engineering overheads present on each disk.

 

This was first published in April 2009

 

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