Answer

Adding an old hard drive to RAID sets on servers

Can I add an old hard drive to RAID sets on servers?

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In answering this question, let’s assume that the old hard drive used to be an internal drive within a particular host or was incorporated within a direct-attached disk enclosure.

If the old hard drive was directly attached to a particular host, for reuse, you will be limited by the number of drive bays available in the receiving host. If only two drive bays are available, the only RAID option that will provide resilience is RAID 1. If three or more bays are available, you begin to open up the possibility of using other RAID levels that will provide a greater amount of usable capacity.

If there is a disk enclosure available, you’ll have more flexibility as to how the old hard drives can be deployed. The first consideration should be whether the enclosure is compatible with the host to which you want to give the additional capacity. If it is, then an expansion slot  must be available to attach the RAID controller for the enclosure. Once this is installed, the drives can be configured in the most appropriate RAID format for the workload.

To configure the drives, first consider whether the new capacity will be used to extend an existing data set, replace an existing data set or create a new data set.

If the capacity is being used to extend an existing data set, it is important to understand whether the OS will stripe the data across the old and new LUNs or whether it will be concatenated. If it will be striped, the new LUN should be the same size and RAID type as the existing LUN, but if it is concatenated, neither of these restrictions need to be adhered to.

If the capacity is being used to replace an existing data set, there will also be performance considerations. If the old data set suffered from poor performance and was RAID 5, it may help to configure the replacement capacity at RAID 10. If performance is not an issue, then it is sensible to keep the new capacity at RAID 5.

This calculation assumes, of course, that there are enough drives to provide the required capacity at RAID 10. If the capacity will be used for a new data set, then the same performance considerations need to be applied as for a replacement data set.

This was first published in November 2011

 

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