Redundant Array of Independent Disks (RAID) Storage Capacity Calculations

Posted by Mike Mahana
When selecting data storage, one of the first decisions you have to make is how much you will need. One of the benefits of Network Attached Storage solutions is their ability to provide plenty of storage but also data redundancy in the case of a disk failure. The primary technology that provides redundancy in NAS solutions is known as RAID (Redundant Array of Independent Disks). In order to decide how many disks you'll need, how much redundancy your configuration will provide, and what your true available storage capacity will be, it is important to understand the difference between the various RAID options.

RAID 0











Capacity Calculation

The capacity of the RAID 0 configuration is limited by the smallest disk.The formula for this configuration is (Total Number of Disks x Smallest Disk Capacity).
Example: (One 500GB disk, One 750GB disk); 2 x 500GB = 1,000GB. As shown by this calculation there is no advantage to having one disk larger than the other(s). 

Key Features
  • High performance as data is written across multiple disks.
  • Requires at least two disks.
  • Data blocks are Striped.
  • No data redundancy (no mirroring, no parity).
  • Should not be used for critical systems.
 RAID 1











Capacity Calculation

The capacity of the RAID 1 configuration is limited by the smallest disk.The formula for this configuration is (Total Number of Disks x Smallest Disk Capacity).
Example: (One 500GB disk, One 750GB disk); 2 x 500GB = 1,000GB. As shown by this calculation there is no advantage to having one disk larger than the other(s).

Key Features
  • Requires at least two disks.
  • Data blocks are not Striped.
  • Good data redundancy as data blocks are mirrored.
  • Configuration can handle one disk failure without data loss per set of two disks installed. 

RAID 5











Capacity Calculation

The capacity of the RAID 5 configuration is limited not only by the smallest disk, but also the space required for the Parity data blocks (P1, P2 and P3). The formula for this configuration is (Total Number of Disks -1) x Smallest Disk.
Example: (Two 500GB disks, One 750GB disk); (3-1) x 500GB = 1,000GB. As shown by this calculation there is no advantage to having one disk larger than the others.

Key Features
  • Requires at least three disk.
  • Data blocks are Striped.
  • Good data redundancy as there is data parity.
  • Configuration can handle one disk failure without data loss per set of three disks installed. 

RAID 6











Capacity Calculation

Like RAID 5, the RAID 6 configuration is limited by the smallest disk as wells as space required for the Parity data blocks (P1, P2 and P3), but also adds a second parity block per disk. The formula for this configuration is (Total Number of Disks -2) x Smallest Disk.
Example: (Three 500GB disks, One 750GB disk); (4-2) x 500GB = 1,000GB. As shown by this calculation there is no advantage to having one disk larger than the others.

Key Features
  • Requires at least four disks.
  • Data blocks are Striped.
  • Excellent data redundancy as there is duel data parity.
  • Configuration can handle up to two disk failures without data loss per set of four disks installed. 

RAID 10 (or RAID 1+0)











Capacity Calculation

RAID 10 combines the mirroring capabilities of RAID 1 with the striping capabilities of RAID 0.The formula for this configuration is (Total Number of Disks x Smallest Disk) x 1/2.
Example: (Two 500GB disks, Two 750GB disks); (4 x 500GB) x 0.5 = 1,000GB. As shown by this calculation there is no advantage to having one disk larger than the others.

Key Features
  • Requires at least four disks.
  • Data blocks are Striped.
  • Excellent data redundancy as disks are mirrored.
  • Configuration can handle up to two disk failures without data loss per set of four disks installed.  

Selecting a RAID configuration is one of the important steps in making selections for your NAS solution. 

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