Articles: Storage

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Solid State Drives

Today, SSDs are rather too expensive. And if you can find an affordable SSD, its characteristics are likely to disappoint: a $100 flash-based drive is going to have a small capacity of 32-64 gigabytes and a rather low performance. The manufactures try to convince us that the cost of storing one gigabyte of data is not the most important factor for SSDs and that we should appreciate such factors as reliability, shock resistance, high IOps per dollar, low temperature, lack of noise, etc. A corporate user will be willing to pay for all this but a home one will be waiting for a lower price.

Both types of users need an individual approach, but the general trend is to replace SLC NAND flash (with 1-bit cells) with MLC NAND flash (with 2-bit and even larger cells). The enterprise MLC type also comes to the market, boasting an increased number of rewrite cycles (6000 instead of 3000). Intel uses such eMLC memory in its new enterprise SSDs. This is all meant to reduce the price of the end product, of course. But SLC-based drives will still be used where the price factor is unimportant, like data warehouses with a huge amount of requests.

In 2009, the cost of storing one gigabyte of data on an enterprise-level SLC-based SSD was up to $20. In 2010, thanks to the release of enterprise-level SSDs based on MLC memory, this cost is as low as $5. These SSDs are intended for servers. SLC-based SSDs are still preferable for large data storage arrays. However, in the next year MLC (or maybe eMLC) based SSDs are expected to span the entire corporate market and replace SLC-based counterparts in many of its sectors, lowering the cost of storage to $5 per 1 gigabyte. As a result, we can expect the price of 1 gigabyte of a consumer SSD with MLC memory to drop to $1 in 2012 (corporate SSDs are going to notch the $1-per-gigabyte mark in 2014).

The following 5-year forecast from Gartner (made in August) shows the development rate of the different sectors of the SSD market.

The majority of solid state drives – about 90 % of the total – are going to target client systems including home desktop and mobile PCs and netbooks. SSDs will be obligatory in top-performance home computers and workstations. They are also going to become an indispensable component of slim notebooks. Less advanced computer systems will either use SSDs optionally or together with HDDs in a hybrid storage system where the OS and applications are loaded from the faster SSD whereas data are stored on the slower HDD. Netbooks will have SSDs by default, but in small quantities. This is Samsung’s vision of the SSD distribution map depending on the cost of the computer system:

Getting back to the Gartner diagram, we can note that the company had lowered its forecast for the SSD market growth. Compared to the July version of the document, the difference is 5 to 10 million units annually. We can suspect that the analysts had corrected their forecast due to the rapid development of tablet PCs which, as we’ve seen above, eat up a share of the PC market. Anyway, the average annual growth of the SSD market will be higher than 70%. This is more than what is expected for tablet PCs, so computer applications of flash memory are still going to be an important factor for the development of NAND flash.

It is especially pleasant for us, home users, that even though consumer SSDs will account for 90% of the total SSD sales, they will only bring their manufactures half the profit. The other half will be provided by the buyers of enterprise SSDs (Gartner).

Now here are two more graphs to show you when the total amount of flash memory is going to exceed the total amount of HDD storage and to evaluate the price reduction rate for SSDs.

The diagram on the left is from IDC. It promises that SSDs will exceed HDDs in terms of the total amount of storage in 2018. The graph on the right, based on Samsung’s data, shows how retail prices on consumer SSDs are going to decline. 64GB SSDs have already crossed the price mark of an average HDD plus $100. 256GB SSDs will cross the +$100 mark in three years. 1TB SSDs will reach that mark in 2016.

While getting cheaper and cheaper, SSDs will also be improving in terms of their consumer properties, which are already quite enticing. The manufacturing technology, interfaces, controllers and the very operation principle of SSDs are all going to be polished off.

Right now, the replacement of SDR with DDR signaling should double the bandwidth as it was with the transition from SDR to DDR SDRAM. As a result, the speed of data transfer will increase from 66 Mbps to 133 Mbps and higher, up to 200 Mbps.

Like with system memory, DDR flash is going to be synchronized with an external DQS signal. There are two possible operation modes: asynchronous (toggle mode) and synchronous. Without delving deep into the technical details, we can note that the first mode is used by Samsung in its newest flash memory chips. The synchronous mode described by the ONFi standard (Open NAND Flash Interface) is promoted by Intel with its partners. DDR flash memory with frequencies up to 200 Mbps is going to be ratified by JEDEC in early 2011. The next step will be to increase the speed of the flash memory interface to 400 Mbps.

Samsung illustrates the benefits for SSDs from transitioning to DDR chips with 133 Mbps and 400 Mbps frequencies.

The number of IOps in random read mode is going to grow by 10% and then by 20% whereas the number of IOps in random write mode by 40 and 60%, respectively. The current generation of NAND flash can only provide the first step of the acceleration. Transitioning to the high-speed 400Mbps interface will call for a larger signal structure and for the introduction of voltage arbitration signals, providing the option of external power supply for memory cells during read and write operations.

The efficiency of the integrated power source of NAND flash, which converts 3.3 volts to 11.5-12.5 volts, is less than 30%, which is especially important for server-oriented SSDs. On-die termination will also have to be introduced. It will be manageable, just like the strobe latency (you can refer to this document for details). The new signals will only add to the existing ones, making the high-speed memory chips backward-compatible with old controllers.

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