Several years ago Adaptec adopted the 5EE RAID level as a standard component of our hardware RAID cards. It seemed like a very good idea – making use of the idle drive that was traditionally a hot spare.
However, as I’ve come to know this RAID level better, I’ve been advising people against using it for one particular reason.
Compare: System 1 – 5 drives in a RAID5 and 1 x hot spare, System 2 – 6 drives in a RAID5EE.
System 2 should run faster than system 1 – after all it has more spindles doing the day to day work … and yes, it does run faster. In theory it’s around 15% faster but I’m yet to see that in practice.
So what’s wrong with this RAID level? My problem lies when a drive dies. With System 1 when a drive dies, the hot spare kicks in, the RAID rebuilds and all is good again.
With System 2, when a drive dies, the RAID5EE compacts itself into a standard RAID5. That’s all good, except when you replace the drive – that’s when I have problem. The RAID5EE will expand itself back out to a RAID5EE – which is another lengthy process which I believe (now) is not required.
So 5EE probably makes sense when you are running just 4 drives in a small JBOD or 1U server, but when the drive count increases I think (again, now) that it’s probably better to run just a RAID5 with a hot spare than to run 5EE.
So is anyone actually using this RAID level and if so, how do you find it?