It seems like the world has stopped turning …

June 3, 2015

No, this is not biblical, nor is it prophetic. In fact I’m referring to disk drives :-)

In a meeting the other day we were discussing unofficial conversations with disk vendors that we have on a regular basis. It seems the spinning world (and I’m talking channel here), is slowing down. The SSD vendors however are romping along at 20%+ growth year on year.

So that is stating the obvious – SSD uptake is growing at the expense of HDD. Of course HDD is still king in the cold data storage world and those guys are making a killing in the datacenter world – all our “cloud” information has to live somewhere (like all your photos of yesterday’s lunch on FB etc).

But in the channel, the SSD is taking over for performance systems. The 10-15K SAS drives are giving way to similar sized SSDs – some at the enterprise level but a lot more at the high-end gaming and channel level – drives that at first glance don’t appear to be made for RAID, but in fact work just fine.

When talking to users, performance is a given – they all understand that the SSD is faster than the spinning drive, but many are still worried about drives wearing out – will they fail? I myself was wondering that so I looked at some specifications of drives and specifically their “DWPD” values (drive writes per day). This is pretty much the standard that SSD vendors use to indicate how long they think a drive will last before the overprovisioning is used up and the drive starts to degrade.

You will see values between 1/3 of a drive write per day and 25 drive writes per day – and if you were using these drives in an intensive write datacenter environment, I know which drives I’d be opting for. But let’s look at the 1/3 drive write per day and do a little maths. Let’s take 4 x 1TB drives (close enough) and make a RAID 10. Roughly speaking, that will give you 2TB capacity. Now if I can safely write 1/3 drive’s worth of data per day for the life of the drive, then that would be (approximately) 600GB of data written each day – remembering that the data is split across the two sets of mirrors in the RAID 10, and each set of mirrors can supposedly handle 300GB per day (1/3 their capacity).

Then lets look at the sort of systems that people are putting SSDs into. Are they using them for video servers? Not likely (too expensive to get the capacity). In fact they are using them for database and high-performance servers that are generally handling lots of small random reads and writes.

A bit more maths works out that an average 40 hour business week is approximately 25% of the overall time during the week, so you’d need to cram that 600GB writes into that timeframe (40 hours) to start stressing the drive. That’s something like 15GB writes per hour … remembering that this is based on a drive with a DWPD of 1/3. So a drive with higher values can handle more, and I’m yet to think of a business that is running SSDs in a database environment that is even within cooee of these numbers.

So when you look at the values on the drives, and think … wow, 1/3 DWPD is not very much … you need to balance that with actually thinking about how much data it is that your business will actually be writing to the disks on any given day.

I found it pretty interesting math – and it opened my eyes to the reality of DWPD values. Remember of course that in a datacenter you should use datacenter drives – mainly because many thousands of users can be accessing those drives at any given point in time and yes, you can get some pretty amazing amounts of data written to the drives each day, but in the channel, in the small business, and even in the medium to large-sized business, the amount of data being written each day is not the scary number that you may have thought without some detailed analysis.

It’s food for thought.

Oh, and by the way, if you are using SSD, then you should be using Series 8 RAID controllers. I know they are 12GB controllers and your SSDs are only 6GB devices, but it’s not the speed of the pipe that matters, it’s the number of transactions per second that the controller can handle. You don’t want to bottleneck your investment in SSDs at the controller level – you want a controller that will easily handle all the small reads and writes that the SSDs are capable of. Now whether your software or customers are capable of throwing or dragging that much data from the drives is a moot point, but putting SSDs on a slow controller is not the smartest thing to do.



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