Answer

Choosing a RAID level depends on capacity needs and data criticality

Which RAID level should be used with three hard disks of 146 GB and one of 300 GB?

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All the main RAID levels are possible, but which RAID level you choose in this case depends on the criticality of the data and the capacity required.

The first thing to note is that when configuring drives in a RAID group, the usable capacity on each drive will always be the same. In this instance, therefore, only 146 GB of the 300 GB drive would be usable in the RAID set.

Before going on to our explanation, here is a matrix of what each RAID level offers us with the drives we have to hand:

RAID 1 is a mirrored pair of drives, so a maximum of two drives is possible. If you add more drives and stripe data across members of the same mirror, it becomes RAID 10, which is a number of RAID 1 pairs in a RAID 0 stripe set.

In our case, RAID 6 is inappropriate. RAID 6 requires two drives for parity and is generally used for large RAID sets made up of high-capacity drives. The rebuild times for high-capacity drives are much longer than for those of low capacity, and RAID 6 ensures data is not lost if there is a second failure during a rebuild operation. In our example above, RAID 6 would require half the available drives for parity. If this level of protection were required, RAID 10 would be much more suitable.

Also, RAID 6 has the same capacity as RAID 10 but is not as quick; RAID 10 can carry out two reads simultaneously whereas RAID 5 and RAID 6 can do only one read at a time. RAID 6 also has to calculate double parity so is much slower. The only case that can be made for RAID 6 is that any two drives can fail and the data will be safe, whereas if two drives that were part of the same mirror in a RAID 10 set failed, then you would lose data.

If you need 500 GB of capacity, you have no choice and must go for RAID 0 as it is the only one without enough capacity. If you require up to 438 GB of capacity, you can go for RAID 0 or RAID 5. If you need less than 292, GB all options are open to you. The capacity you need may discount some of the RAID levels as insufficient unless you purchase more disks.

So, to sum up, if performance is the priority, then RAID 0 or RAID 10 should be used. RAID 0 should be used where fault tolerance is not necessary, and RAID 10 should be used where it is. If a balance between performance and capacity is needed, then RAID 5 should be used. 

This was first published in July 2011

 

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