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Excellent summary. One thing he left off: some SSDs continue to copy/erase blocks even if there is nothing new to write because multi-level cell state does degr
by 0xff00ffee 7y ago
Excellent summary. One thing he left off: some SSDs continue to copy/erase blocks even if there is nothing new to write because multi-level cell state does degrade over time. There is a concern that some MLC drives will suffer bit corruption over time if not regularly power up to allow this in the background. Citation needed: I only recall this when I was interviewing to work for Western Digital many years ago.
- wtallis 7y agoThis problem was most prominent right before the switch to 3D NAND, when planar NAND dimensions were at their smallest and the consumer market had already mostly switched over to 3 bit per cell TLC rather than 2bpc MLC. In the worst case, we were down to about 8 electrons difference between cell voltage states. That's now been relaxed by 3D NAND allowing for larger cell sizes, and most 3D NAND also switched from floating-gate to charge-trap cell design so leakage is less of an issue. Nowadays, data retention in SSDs is only a concern toward the end of their lifespan (as measured by write endurance), and it's probably inadvisable for the SSD to start doing background data scrubbing until the raw read error rate starts climbing.
- 0xff00ffee 7y agoThank you for bringing me up to speed!