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Memory Changes Everything July 12, 2010

Posted by mwidlake in Architecture, performance.
Tags: , , ,
9 comments

I’ve got this USB memory stick which I use to carry around my scripts, documents, presentations, Oracle manuals and enough music to keep me going for a few days. It is on an 8GB Gizzmo Junior and it is tiny. By tiny I mean as wide as my little finger, the length of a matchstick and about the same thickness of said matchstick. So small that I did indeed lose the damn thing for 6 months before I realised it had got trapped behind a credit card in my wallet.

It cost me ten British pounds about 15 months ago (less than most 4GB USB sticks seem to cost now, but then it is nothing more than the memory chip and connectors wrapped in plastic) and it highlights how cheap solid-state “storage” is becoming.

Connected to this, I was looking at buying a new PC this week and this machine comes with 10 USB slots, if you include the ones on the supplied monitor and stubs on the motherboard.
10 USB slots, 8GB gizzmo memory sticks… That would be 80GB of cheap and fast storage. Now get a few USB hubs and bulk-buy a few dozen cheap USB2 sticks and you could soon have a solid-state database of a few hundred GB for a thousand pounds. Then of course you can have fun seeing where the pinch-points in the system are (USB2 has a maximum speed per port and going USB3 right now is going to break that 1 grand barrier. But give it a year…).

This really started me thinking about when memory-based storage would take over from spinning disk as the best option for enterprise-level storage and my gut feeling is in about 5 years. I think it will be both technically possible and financially viable in much less than that, say as little as 2 years, but the cost of solid-state storage per MB will still be higher than disk by then but potentially much faster. A few considerations going through my mind were:-

  • Disk is getting a lot slower in relation to acreage. By this I mean that, for a single disc drive, capacity is doubling about every 18 months but seek time has hardly reduced in a decade and transfer rate (reading from the physical platters to the units buffer) is again almost stationary, at about 120MB/s for 10,000rpm disk and up towards 180 for those very expensive and noisy 15,000 rpm disks. Being a tad ridiculous to make the point, with modern 3TB disks you could build most Oracle database on one disc. Let’s make it two in a raid 10 configuration for redundancy. My point is, your 3TB database could well be being run right now, for real, across say 5 physical disks with a total sustainable physical throughput of around 500MB a second.
  • Solid state storage seems to be halving in price in more like 8-10 months.
  • IO subsystems are made faster by using RAID so that several physical discs can contribute to get towards the 300MB or so speed of the interface – but solid state is already that fast.
  • IO subsystems are made faster by building big caches into them and pre-fetching data that “might” be requested next. Oh, that is kind of solid state storage already.
  • Solid state storage, at least the cheap stuff in your USB stick, has the problem that you can only write to each bit a thousand or so times before it starts to get unreliable. But physical disk has exactly the same issue.
  • There are new methods of solid-state memory storage coming along – “New Scientist” had a nice article on it a few months ago, and these versions will be even higher density and more long-term reliable.
  • Seek time on solid-state memory is virtually zero, so random IO is going to be particularly fast compared to spinning disk.

Solid state memory needs less power, and thus less cooling, is silent, is potentially denser and is less vulnerable to temperature and humidity fluctuations. I can see it not needing to be kept in a specialist server room with the need for all that air con and ear defenders when you go in the room.
Just somewhere with normal air con and a lock on the door should suffice.
We do not need Solid State storage to match the size of current disks or even be as cheap to take over. As I have already pointed out, it is not acreage you need with physical disks but enough spindles and caches to make it fast enough in relation to the space. Further, we can afford to pay more for solid state if we do not need to keep it in such expensive clean-room like environments.

I can see that in a couple of years for a given computer system, say a mixed-workload order processing system, to support the storage needs we will have maybe a dozen solid-state chunks of storage, perhaps themselves consisting of several small units of memory in some sort of raid for resilience, all able to flood the IO channels into our processing server and the issue will be getting the network and io channels into the server to go fast enough. So don’t, stick all the storage directly into the server. You just got rid of half your SAN considerations.

I’m going to stop there. Partly because I have run out of time and partly because, in checking out what I am writing, I’ve just spotted someone did a better job of this before me. Over to James Morle who did a fantastic post on this very topic back in May. Stupid me for not checking out his blog more often. Jame also mentions that often it is not total throughput you are interested in at all but IOPS. That zero latency of solid-state memory is going to be great for supporting very high IOPS.

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