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MLC vs SLC: Which flash SSD is right for you?

Which flash SSD you choose depends on the performance you need and the price you want to pay, but the differences are not as great as you may think

MLC versus SLC: It’s a difference you need to know about when thinking of buying solid-state disks (SSDs) based on flash memory technology, which is now affordable enough to merit serious consideration when planning a storage system.

As with any technology, there are trade-offs, depending on which of the two types of flash SSD you select. Multi-level cell (MLC) flash is most common and is often found in consumer-grade products such as cameras, phones, USB memory sticks and portable music players, but it is also present in some enterprise storage products.

The main characteristic of MLC flash is its low price, but it suffers from higher wear rates and lower write performance compared with single-level cell (SLC) technology. SLC is faster and much more reliable – but also more expensive – and is featured in the best-performing storage arrays.

In practice, however, the differences are not quite as clear as you may expect. To see how this technology is developing, its application and where it is heading, we need to look at how the two types of flash memory work and how they are sold.

Storage sales discussions are not normally about the trade-offs of MLC vs SLC, said Valdis Filks, storage technologies director at research firm Gartner. “This is normally hidden by implementation,” he said. “In other words, it’s up to the enclosure manufacturer of the storage array, and it’s the controller that’s more important than the underlying storage technology.”

MLC vs SLC head to head

Suppliers may prefer not to discuss the differences between the technologies, but understanding the underlying technology can influence deployment strategies. So, what are the key differences between MLC and SLC flash SSD?

All flash memory suffers from wear, which occurs because erasing or programming a cell subjects it to wear due to the voltage applied. Each time this happens, a charge is trapped in the transistor’s gate dielectric and causes a permanent shift in the cell’s characteristics, which, after a number of cycles, manifests as a failed cell.

SLC uses a single cell to store one bit of data. MLC memory is more complex and can interpret four digital states from a signal stored in a single cell. This makes it denser for a given area and so cheaper to produce, but it wears out faster.

So, an MLC cell is typically rated at 10,000 erase/write cycles, while an SLC cell might last 10 times that before failing. However, manufacturers of products consisting of MLC cells can and do have ameliorating technologies and techniques at their disposal.

According to Andrew Buss, service director at analyst firm Freeform Dynamics, amelioration techniques used by most suppliers include wear-levelling, which moves write cycles around the chip so that cells wear evenly; on-device deduplication, which reduces the volumes of data written and so lowers wear; redundancy, which reserves a portion of the device’s capacity to replace cells as they fail; and write optimisation, which stores data writes so they can be made in large chunks to reduce the number of write operations. The emerging term for MLC products that incorporate such techniques is enterprise MLC, or eMLC.

Most such techniques are implemented in the device controller – the interface between device and computer –with companies such as SandForce and Intel among the most advanced in implementing such techniques, according to Buss. And despite the endurance issues related to SSDs, they remain, say suppliers, more reliable than spinning media.

Vendor market share

STEC, which sells 49.9% of all SSDs globally, according to Gartner, was the SSD product market leader in 2009 (Gartner was unable to break out UK figures). Fusion-io, Intel, Texas Memory Systems, Samsung and Sun Microsystems follow with shares all below 9%. All the main storage array suppliers include SSDs in their product offerings, pitching them as Tier 1 or Tier 0 in policy-managed tiered storage systems.

According to Filks, the implementation determines the technology. So applications such as high-speed databases, whose performance is measured in terms of transactions per second, should be matched to the appropriate technology selected on the basis of price/performance.

“It’s about serving more customers in a given time – that’s what SSD vendors talk about,” said Filks.

Price difference

Despite this, MLC and SLC tend to find themselves used for different applications, due largely to the four-fold price difference per gigabyte between them. As we have seen, MLC can be found in consumer-grade products, but also in the enterprise where performance, although important, is not the primary consideration.

When used in the same storage system, the two types of SSD can be tiered in the same way as tiering with spinning media; most storage product suppliers include a form of automated SSD tiering, according to Buss. SLC typically tops the storage tier tree in financial services organisations, where high-speed access to large databases is essential and price is a secondary issue. Buss said he sees future products increasingly integrating both flash SSD types and spinning media in performance/cost-based tiers.

“Most enterprise applications will rely on a form of database and so will need SSDs,” said Buss. “An example is content management systems, where an end-user is waiting for things to happen; also Exchange servers, websites, media storage – all of which you can use MLC for. However, you still need to do due diligence and buy appropriately. There are new solutions coming along to make MLC better.”

End-user Roger Bearpark, assistant head of ICT at the London Borough of Hillingdon, has installed 520GB of MLC-based SSD-based storage into his Compellent arrays. He said: “MLC is poorer on endurance and performance, but is up to three or four [times] better on price. We got a phenomenal rate of return on investment by putting small amounts of active data on SSD, which produced a 13-fold improvement in access times.”

The future

According to Filks, SSDs will not replace spinning disks. “Everyone says SSDs will replace disks – maybe in about 15 to 20 years’ time – but as SSD prices drop, so do those of disks,” he said. “And SSD prices will never fall as far as disk because factories can’t make enough. It means only the working dataset needs to be on SSD, and that’s about 5-15% of the total.”

However, Filks predicted that SSD could eventually replace tape as a deep archive technology because it offers similar benefits – non-volatility and zero power usage when not in use – although this will take 10 to 20 years.

As prices fall and reliability techniques improve, it seems likely that MLC technology’s price advantage will see it stay ahead of SLC for all but the most demanding of applications, as it remains significantly faster and more robust than spinning media.

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