What Is Hot Spare and How Does It Work?

If you’re familiar with using file servers, you’re probably familiar with using RAID. Most file servers run on RAID 5, and offer a good degree of speed as well as a layer of protection from losing any data. One hard disk in a RAID 5 set up may completely fail, still without any data loss occurring. This is because RAID 5 uses a smart system, confusingly known as ‘parity’ that allows it to work out and replicate what data is missing from the lost disk.

But once a disk fails on a RAID 5 system, that system is no longer running at it’s optimum level: data transfer speeds suffer as the RAID 5 has to work a lot harder to preserve the data across the remaining disks. This is known as the RAID operating in ‘degraded’ mode. On busy servers, or where there is a watchful Systems Administrator who will notice that a RAID has gone into degraded mode it’s a simple enough job to swap the faulty disk with a new one. But on systems where there is no one keeping an eye on events, a RAID 5 can be running in degraded mode for months, and this is only noticed when another disk in the array fails and then the whole server crashes.

By using what is known as a ‘hot spare’, a RAID 5 can automatically swap out a failed drive and replace it with a new one. This new replacement drive is known as a hot spare. Once a defective drive in the RAID is detected, it is automatically removed from being part of the RAID array and the new drive is introduced into the array in its place. This action then leads to the automatic ‘rebuilding’ of the array – the process of including the new drive into the existing RAID set of drives. Rebuilding can take anything from about 20 minutes to several hours depending on how much data their is and the state of the hard drives in the array. If you need RAID data recovery please click.

It’s always a good idea to keep all the hard drives in a RAID in a healthy state and check them for bad sectors, replacing any drives that fail the test with new ones.