Information for Business from Lenovo
Data Centre + Big Data
Contributor: Graeme Philipson
Why SSD is replacing rotational storage

The days of the disk drive are numbered. Spinning disks – also known as rotational storage – are in decline and will disappear in the next decade or so. They are being replaced by solid-state drives (SSDs), which are faster, smaller, quieter and use less power. To understand why, let’s look briefly at the history of computer storage.

SSDs have been around for a while, but they have always been more expensive than disks. Now the price differential is much smaller, and being eroded by the many advantages of the SSD.

This is a good thing for many reasons. The performance improvements are substantial, and in an era where energy efficiency is a major factor in hardware design, the lower power consumption of SSDs is a major advantage.

Disk drives were invented by IBM in 1956 – nearly 60 years ago. In computing terms, that is an eternity. Since then, they have continuously improved in performance and dropped in price, like most computer hardware.

No one has given their name to the phenomenon, unlike Moore’s Law, which describes improvements in processing power. If anything, the improvements in disk performance over the years have been even more remarkable.

Storage has come a long way

The first disk drives were giant clunky things with 50 platters, each bigger than a dinner plate, and a total storage capacity of less than 4 megabytes. By the 1970s they had reached 100 megabytes, in large heavy disk packs that had to be mounted manually by computer operators (I know – I was one of them).

When PCs came out in the early 1980s, they did not have hard disks – offline storage meant cassette tapes or floppy disks. As a young computer salesman, I personally sold the first ever IBM PC with a hard disk drive in Australia – the PC XT. The year was 1983, and it had a 10 megabyte hard disk that added $5000 to the price of a floppy-only IBM PC.

Today, entry-level PCs routinely have hard disk drives of 500 gigabytes, or even a terabyte. These retail separately at less than $100 – a staggering improvement in price/performance in less than 30 years.

But the days of the disk drive are coming to an end. Just as they have been constantly improving, so has solid-state storage.

ILM and the storage hierarchy

There is an important concept in computing known as the ‘storage hierarchy’ or ‘memory hierarchy’. There are a many different types of computer storage, delineated by speed and cost. The fastest, but most expensive, is contained within the central processing unit (CPU), which shuffles small amounts of data in and out at very high speeds, rated in gigahertz.

Then comes cache memory – data that has just been accessed or is just about to be accessed by the CPU. Nowadays, with high-density manufacturing technology, cache memory is usually contained on the same chip as the CPU. Then comes RAM (random access memory), followed by disk drives, followed by tape.

The closer the memory is to the CPU, the faster it can be accessed, and the more it costs. Information life cycle management (ILM) is the process of juggling memory between the different levels of the hierarchy. ILM is a dynamic trade-off between cost and speed of access. It is a major computer discipline in its own right.

Bucking the trend

Solid-state drives are a comparatively recent addition to the storage hierarchy. IBM, ever the pioneer, introduced memory caches in mainframe computers in the 1970s. They were similar to the RAM used in computer cores, but they looked like disk drives to the system. They were much more expensive, but they were super-fast.

The idea caught on, and what came to be called solid-state drives became commonplace. They were always much more expensive – as soon as their price dropped, so did that of disk drives – but the price/performance calculations of ILM meant that they had an increasingly important role to play.

Narrowing the price/performance ratio

Now the price/performance improvements of HDDs and SSDs are converging. Disk is still cheaper than solid-state, but both are so cheap that the price differential is becoming irrelevant – look at USB memory sticks.

For years, people have forecast the eventual demise of the disk drive for the simple reason that its moving parts are inherently inefficient. But not everyone holds this view because disks have continued to improve, always staying one step ahead of solid-state.

Things are now starting to change. Many laptop computers now come with only SSD, and tablets and mobile phones have always had solid-state drives.

The death knell for HDD

The trend is continuing. It is not enough for rotational storage to continue to improve in price/performance. It is sufficient for solid-state storage to become affordable, such that the price differential between the two becomes less important than the advantages of SSD. That time is now.

Solid-state has always been faster, lighter and less demanding of electrical power than rotational. Now, in the age of portable digital gadgets, those advantages are proving to be more than enough to make the future of rotational look shaky.

Each generation of storage technology, as it drops in price, moves towards the lower end archival part of the storage spectrum. That is where disk drives are headed, as solid-state drives become the norm.

This is as true for the data centre as the PC. Solid-state is now commonplace in server rooms and data centres as the price/performance continues to improve.

SSD is already the norm in many enterprises for frontline operational work. Slower, bulkier and energy-hungry rotational disk drives are increasingly being relegated to backup and archival work. Eventually they will disappear even from that role.

The future is solid-state, from mobile device to data centre.


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