Wear leveling in SSDs considered harmful

Abstract

We argue that wear leveling in SSDs does more harm than good under modern settings where the endurance limit is in the hundreds. To support this claim, we evaluate existing wear leveling techniques and show that they exhibit anomalous behaviors and produce a high write amplification. These findings are consistent with a recent large-scale field study on the operational characteristics of SSDs. We discuss the option of forgoing wear leveling and instead adopting capacity variance in SSDs, and show that the capacity variance extends the lifetime of the SSD by up to 2.94×.

On the Cost of Near-Perfect Wear Leveling in Flash-Based SSDs

“…The downside of any WL technique is that it causes additional internal write operations, thereby increasing the so-called write amplification (WA) factor, which is defined as the ratio between the total number of writes performed by the SSD and the number of writes requested by the host system. In a recent paper [13] the increase in the WA factor was shown to be very substantial for a number of representative WL algorithms making the authors question the usefulness of wear leveling in modern SSDs.…”

“…Blocks containing mostly cold pages on the other hand are far less regularly selected as it takes a long time before its pages become invalid. Thus any WL technique needs to make sure that the cold pages also migrate regularly (e.g., by performing periodic swaps between blocks), but this clearly comes at the expense of an increase in the WA as demonstrated in [13] for a number of representative WL algorithms.…”