LSI boosts SSD capacities with over-provisioning change

LSI has confirmed that it is finally allowing its solid-state controller customers to disable the extra over-provisioning normally associated with its SandForce parts in order to boost storage capacities at little cost.

Over-provisioning in solid-state drives (SSDs) is the act of 'stealing' a portion of the flash storage for internal use. While the flash still gets used, it's invisible to the user and unavailable for general storage.

All SSDs use over-provisioning to some extent, but SandForce controllers demanded more: where around 7 per cent was standard, consumer-grade SandForce-based drives demand over 14 per cent while enterprise-grade versions reserved even more.

It's easy to see the difference it makes: where SandForce drives are available in capacities of 60GB, 120GB, 240GB and so on, rival non-SandForce drives typically offered 64GB, 128GB, and 256GB at the same price point. By removing this extra over-provisioning, LSI is able to allow its SandForce customers to release drives with the same capacity as their rivals.

A drop in over-provisioning doesn't come for free, however. Performance can take a severe hit when over-provisioning is disabled on drives that support such changes, with manufacturers sometimes having to add DRAM-based cache to the drives to compensate. LSI claims, however, that the change has little to no effect on performance or longevity.

Speaking to The SSD Review, senior director of product management at LSI's flash division Kent Smith explained that the controller still offers a chunk of over-provisioning hidden by the difference between 'real' gigabytes - more properly called 'gibibytes,' or 'binary gigabytes' - and 'manufacturers' gigabytes.

It's no secret that hard-drive manufacturers have been playing bait-and-switch with capacities for years. Where a gigabyte in computing terms is equal to 1,024 megabytes - and a megabyte to 1,024 kilobytes, and so forth - manufacturers use the rounder 1,000 megabyte figure. As a result, as the drive gets bigger you lose a chunk of space to what are essentially rounding errors. A '120GB' hard drive, as an example, will only present 111.76GB to the host operating system.

This difference is roughly equal to 7 per cent of the total drive capacity. That allows LSI, Smith explains, to drop to 'zero per cent' over-provisioning through a firmware upgrade to its SandForce controller despite having a practical lower limit of 7 per cent over-provisioning as part of its innovative controller design.

Potentially, LSI admits, the change will come at a slight drop in performance compared to a traditional SandForce controller. For those looking for a storage boost, however, that could be a trade-off worth making.

The first manufacturer out of the blocks with a product based around the new firmware is ADATA with its XPG SX900, which will be available soon in 64GB, 128GB, 256GB and 512GB capacities. Thus far, no manufacturer has suggested that the firmware will be available as an aftermarket upgrade for existing drives.