On Thu, Apr 19, 2018 at 01:39:04AM -0700, Christoph Hellwig wrote: > On Wed, Apr 18, 2018 at 12:52:19PM -0400, J. Bruce Fields wrote: > > > Theodore Y. Ts'o - 10.04.18, 20:43: > > > > First of all, what storage devices will do when they hit an exception > > > > condition is quite non-deterministic. For example, the vast majority > > > > of SSD's are not power fail certified. What this means is that if > > > > they suffer a power drop while they are doing a GC, it is quite > > > > possible for data written six months ago to be lost as a result. The > > > > LBA could potentialy be far, far away from any LBA's that were > > > > recently written, and there could have been multiple CACHE FLUSH > > > > operations in the since the LBA in question was last written six > > > > months ago. No matter; for a consumer-grade SSD, it's possible for > > > > that LBA to be trashed after an unexpected power drop. > > > > Pointers to documentation or papers or anything? The only google > > results I can find for "power fail certified" are your posts. > > > > I've always been confused by SSD power-loss protection, as nobody seems > > completely clear whether it's a safety or a performance feature. > > Devices from reputable vendors should always be power fail safe, bugs > notwithstanding. What power-loss protection in marketing slides usually > means is that an SSD has a non-volatile write cache. That is once a > write is ACKed data is persisted and no additional cache flush needs to > be sent. This is a feature only available in expensive eterprise SSDs > as the required capacitors are expensive. Cheaper consumer or boot > driver SSDs have a volatile write cache, that is we need to do a > separate cache flush to persist data (REQ_OP_FLUSH in Linux). But > a reasonable implementation of those still won't corrupt previously > written data, they will just lose the volatile write cache that hasn't > been flushed. Occasional bugs, bad actors or other issues might still > happen. Thanks! That was my understanding too. But then the name is terrible. As is all the vendor documentation I can find: https://insights.samsung.com/2016/03/22/power-loss-protection-how-ssds-are-protecting-data-integrity-white-paper/ "Power loss protection is a critical aspect of ensuring data integrity, especially in servers or data centers." https://www.intel.com/content/.../ssd-320-series-power-loss-data-protection-brief.pdf "Data safety features prepare for unexpected power-loss and protect system and user data." Why do they all neglect to mention that their consumer drives are also perfectly capable of well-defined behavior after power loss, just at the expense of flush performance? It's ridiculously confusing. --b.
