Libra: Software-Controlled Cell Bit-Density to Balance Wear in NAND Flash

Hybrid flash storages combine a small Single-Level Cell (SLC) partition with a large Multilevel Cell (MLC) partition. Compared to MLC-only solutions, the SLC partition exploits fast and short local write updates, while the MLC part brings large capacity. On the whole, hybrid storage achieves a tangible performance improvement for a moderate extra cost. Yet, device lifetime is an important aspect often overlooked: in a hybrid system, a large ratio of writes may be directed to the small SLC partition, thus generating a local stress that could exhaust the SLC lifetime significantly sooner than the MLC partition's. To address this issue, we propose Libra, which builds on flash storage made solely of MLC flash and uses the memory devices in SLC mode when appropriate; that is, we exploit the fact that writing a single bit per cell in an MLC provides characteristics close to those of an ordinary SLC. In our scheme, the cell bit-density of a block can be decided dynamically by the flash controller, and the physical location of the SLC partition can now be moved around the whole device, balancing wear across it. This article provides a thorough analysis and characterization of the SLC mode for MLCs and gives evidence that the inherent flexibility provided by Libra simplifies considerably the stress balance on the device. Overall, our technique improves lifetime by up to one order of magnitude at no cost when compared to any hybrid storage that relies on a static SLC-MLC partitioning.