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How does the RAID 5 calculation work?

I need to configure a server as RAID 5. The server has eight 146 GB drives. I want to make the C drive 20 GB, the G drive 75 GB and give the remaining space to the D drive. What’s the RAID 5 calculation to determine the capacity of the D drive?

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RAID parity technologies such as RAID 5 require extra disk space to store parity data. In the event of a drive failure, the disk array can use this RAID parity data to rebuild the lost data onto a new drive. RAID 5 needs one additional drive per RAID group for parity information, although the RAID parity data is spread among all the disks.

If we take the eight drives available, one drive’s worth of capacity will be taken up by the parity overhead, leaving seven 146 GB drives -- a total of 1,022 GB -- available for data.

But the RAID 5 calculation doesn’t stop there. You should also consider that the amount of data available to the operating system will never be the amount designated on the disk drive. When a RAID group is created on a storage array, all the drives are formatted in preparation for the creation of LUNs. This operation reduces the capacity of the RAID group by around 5% to 10%.

Following a LUN being presented to a host, the operating system also carries out a formatting operation that can further reduce the usable amount. There are also different ways in which gigabytes can be represented within devices: Some use a multiplying factor of 1,000 between units such as bytes, kilobytes, megabytes, gigabytes, etc., with other devices using a factor of 1,024. The way the operating system and array hardware calculate the multiplying factor will alter the amount of usable space you see on the operating system.

A simple rule for RAID 5 calculation is to take the amount of capacity on the disk drive (in this case 146 GB) and reduce it by about 15% to get an idea of the usable amount that will be available to hosts. Therefore, in the case of this example, a reasonable usable amount of capacity in our RAID group would be 868 GB (1,022 GB minus 15%). The amount available for the D drive would therefore be 773 GB (868 minus 20 for the C drive and 75 for the G drive).

This was first published in February 2011

 

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