On Thu, 18 Jun 2020, Damien Le Moal wrote: > Are you experiencing data loss or corruption ? If yes, since a clean reboot or > shutdown issues a synchronize cache to all devices, a corruption would mean that > your SSD is probably not correctly processing flush cache commands. Cache flushes do not matter that much when SSDs and sudden power cuts are involved. Power cuts at the wrong time harm the FLASH itself, it is not about still-in-flight data. Keep in mind that SSDs do a _lot_ of background writing, and power cuts during a FLASH write or erase can cause from weakened cells, to much larger damage. It is possible to harden the chip or the design against this, but it is *expensive*. And even if warded off by hardening and no FLASH damage happens, an erase/program cycle must be done on the whole erase block to clean up the incomplete program cycle. Due to this background activity, an unexpected power cut could damage data *anywhere* in an SSD: it could hit some filesystem area that was being scrubbed in background by the SSD, or internal SSD metadata. So, you want that SSD to know it must be quiescent-for-poweroff for *real* before you allow the system to do anything that could power it off. And, as I have found out the hard way years ago, you also want to give the SSD enough *extra* time to actually quiesce, even if it claims to be already prepared for poweroff [1]. When you do not follow these rules, well, excellent datacenter-class SSDs have super-capacitor power banks that actually work. Most SSDs do not, although they hopefully came a long way and hopefully modern SSDs are not as easily to brick as they were reported to be three or four years ago. [1] I have long lost the will and energy to pursue this, so *this* is a throw-away anecdote for anyone that cares: I reported here a few years ago that many models of *SATA* based SSDs from Crucial/Micron, Samsung and Intel were complaining (through their SMART attributes) that Linux was causing unsafe shutdowns. https://lkml.org/lkml/2017/4/10/1181 TL;DR: wait one *extra* second after the SSD acknowleged the STOP command as complete before you trust the SSD device is safe to be powered down (i.e. before reboot, suspend, poweroff/shutdown, and device removal/detach). This worked around the issue for every vendor and model of SSD we tested. -- Henrique Holschuh
