When the small things in life matter …

October 19, 2011

Quite some time ago Adaptec released a technology called “Hybrid RAID”. This is the combination of an SSD and a spinning disk in a RAID1 or RAID10 array.

Initially it was a little unclear as to who would use this technology, with gamers, workstations and entry-level servers seeming like the immediate candidates. However as we have progressed with this technology it has become clear that the top end of town (datacenters and corporate) are extremely interested in this technology. It takes a little bit of an explanation to see why …

If you are building a 16-drive server, and want to put the OS on fast disks (eg SSD), there are not many users who will run their server boot drives on just one disk – therefore a mirror is generally the accepted way of protecting this portion of the data puzzle on a server (no matter whether datacenter or home user).

The problem becomes one of capacity and slots. If you use up two slots for SSD for your boot drive, then you only have (in this example), 14 drive slots left for data storage.

So lets look at some maths.

Server 1
2 x 30Gb SSD drives for OS
14 x 2Tb drives for data
(for this example to keep the maths easy we’ll use say that 2Tb drives are in fact 2000Gb – which of course they are not)

Server 2
1 x 30Gb SSD drives for OS
15 x 2Tb drives for data

Capacity of Server 1 is:
30Gb for OS
13 x 2000Gb for data (losing one disk for RAID5 parity) = 26000Gb

Capacity of Server 2 is:
30Gb for OS
(this is made of 30Gb from both the SSD and 30Gb from the first 2000Gb hard drive – leaving 1970Gb usable space left on that drive)
14 x 1970Gb for data (losing one disk for RAID5 parity) = 27580Gb

That’s a 6.08% increase in capacity.

Of course, you could always make a RAID5 array of the 30Gb chunks not used on 14 of the hard drives, which would give an additional 420Gb of space – might be handy for swap files etc but we’ll discount this usage for the moment.

Server 1 costs … 2 x 30Gb SSD + 14 x 2Tb HDD
Server 2 costs … 1 x 30Gb SSD + 15 x 2Tb HDD

Just looking at Google for pricing indicates that server 2 will be cheaper than server 1, especially if a good SLC SSD is used for the boot drive.

Now a 6.08% increase in capacity might not sound like much if you are just buying one server, but when you are tasked with purchasing large amounts of data, this makes a big difference. If you can reduce the number of physical servers in your datacenter you reduce rack space, running costs, cooling costs – everything is impacted by having less hardware in your datacenter.

As far as cost is concerned, this can add up to massive savings for large organisations. Server is cheaper in the first place, less serves required, less servers to power, less cooling costs associated with fewer servers – it’s win, win, win when it comes to the financial side of these calculations.

Now that’s a smart use for this simple technology.



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