Over provisioning is one of the most common ways that SSD designers can help assure that an SSD has a longer life than the flash’s endurance rating would support. If an SSD contains more flash than is presented at its interface, the controller can manage wear across a larger number of blocks while at the same time accelerating disk performance by moving slow operations like block erases out of the way of the SSD’s key functions.
Many people like to compare wear leveling to rotating a car’s tires. In this vein, think of over provisioning as having a bunch of spare tires (rather than just one) in the trunk of your car. The more tires you have, the farther you can drive before having to buy a new set of tires, although at any one time you are never using more than four tires. On the other hand, this pushes up the cost of a tire change a good bit.
Over provisioning also helps reduce write amplification. When an SSD gets full the garbage collection algorithm works to consolidate partial blocks to provide more free space. An SSD with a lot of spare blocks doesn’t need to go through this process as frequently as an SSD with few spare blocks.
(A related, but somewhat different way that blocks are freed is through the Trim command, which will be described in a future post. The Trim command allows the operating system to tell the SSD which sectors are no longer useful. This allows the SSD to erase unused blocks well in advance of their use, reducing the likelihood that the SSD will fill up.)
How much does over provisioning help? Based on inputs from a few companies The SSD Guy concludes that, for standard MLC NAND flash, 45% over provisioning will give you about twice the drive life of 20% over provisioning, and 75% over provisioning will extend the disk’s life to three times. A full 100% over provisioning won’t get you quite as far as four times the drive life, but it comes close. Increases in over provisioning show a trend of diminishing returns approaching zero at around 250% over provisioning (and about 6 times the life of that 20% over provisioned SSD.)
Many contend that 7% over provisioning is a standard for client SSDs. This level would provide about 1/3rd of the lifetime of a 20% over provisioned SSD.
Something very weird happens with compression, which is used in SandForce controllers. These controllers compress the data as it is being written into the NAND flash. This means that the amount of over provisioning is a function of how compressible the data is. Highly-compressible data will leave a lot of flash unused for data storage, so the SSD will have a very large amount of over provisioning, extending its wear significantly. Incompressible data (usually data that has already been compressed, like MP3 files, JPEG photos, or video files) will consume most of the flash in the SSD leaving very little for over provisioning. SSDs storing incompressible data will wear out after fewer disk writes. Quite fortunately, applications with high write workloads do not usually manage these file types.
A note about the terminology: Some folks spell this as a single word – Overprovisioning – and even The SSD Guy is inconsistent about which way to write it. Since it’s a term that began its days decades ago with RAID you would expect some standard to already be in place, but this seems not to be the case. I suggest picking one that feels right to you and overlooking the inconsistency across the industry.
This post is part of a series published by The SSD Guy in September-November 2012 to describe the leading methods SSD architects use to get the longest life out of an SSD despite the limited number of erase/write cycles that NAND flash specifications guarantee. The following list provides the names of all of these articles, and hot links to them:
- Wear Leveling
- External Data Buffering
- Improved ECC
- Other Error Management
- Reduced Write Amplification
- Over Provisioning
- Feedback on Block Wear
- Internal NAND Management
Click on any of the above links to learn about how each of these techniques works.
Alternatively, you can visit the Storage Networking Industry Association (SNIA) website to download the entire series as a 20-page booklet in pdf format.