On Mon 2017-05-08 08:21:34, David Woodhouse wrote: > On Sun, 2017-05-07 at 22:40 +0200, Pavel Machek wrote: > > > > NOTE: unclean SSD power-offs are dangerous and may brick the device in > > > > the worst case, or otherwise harm it (reduce longevity, damage flash > > > > blocks). It is also not impossible to get data corruption. > > > > > I get that the incrementing counters might not be pretty but I'm a bit > > > skeptical about this being an actual issue. Because if that were > > > true, the device would be bricking itself from any sort of power > > > losses be that an actual power loss, battery rundown or hard power off > > > after crash. > > > > And that's exactly what users see. If you do enough power fails on a > > SSD, you usually brick it, some die sooner than others. There was some > > test results published, some are here > > http://lkcl.net/reports/ssd_analysis.html, I believe I seen some > > others too. > > > > It is very hard for a NAND to work reliably in face of power > > failures. In fact, not even Linux MTD + UBIFS works well in that > > regards. See > > http://www.linux-mtd.infradead.org/faq/ubi.html. (Unfortunately, its > > down now?!). If we can't get it right, do you believe SSD manufactures > > do? > > > > [Issue is, if you powerdown during erase, you get "weakly erased" > > page, which will contain expected 0xff's, but you'll get bitflips > > there quickly. Similar issue exists for writes. It is solveable in > > software, just hard and slow... and we don't do it.] > > It's not that hard. We certainly do it in JFFS2. I was fairly sure that > it was also part of the design considerations for UBI — it really ought > to be right there too. I'm less sure about UBIFS but I would have > expected it to be OK. Are you sure you have it right in JFFS2? Do you journal block erases? Apparently, that was pretty much non-issue on older flashes. https://web-beta.archive.org/web/20160923094716/http://www.linux-mtd.infradead.org:80/doc/ubifs.html#L_unstable_bits > SSDs however are often crap; power fail those at your peril. And of > course there's nothing you can do when they do fail, whereas we accept > patches for things which are implemented in Linux. Agreed. If the SSD indiciates unexpected powerdown, it is a problem and we need to fix it. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
