On Mon, 6 Jan 2020 at 16:42, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Mon, Jan 06, 2020 at 05:40:20PM +0200, Amir Goldstein wrote: > > On Mon, Jan 6, 2020 at 9:26 AM Sitsofe Wheeler <sitsofe@xxxxxxxxx> wrote: > > > If a write occurs on one or two bytes of a file at about the same time as a power > > > loss, are other bytes of the file guaranteed to be unchanged after reboot? > > > Or might some other bytes within the same sector have been modified as well? > > > > I don't see how other bytes could change in this scenario, but I don't > > know if the > > hardware provides this guarantee. Maybe someone else knows the answer. > > The question is nonsense because there is no way to write less than one > sector to a hardware device, by definition. So, treating this question > as being a read-modify-write of a single sector (assuming the "two bytes" > don't cross a sector boundary): > > Hardware vendors are reluctant to provide this guarantee, but it's > essential to constructing a reliable storage system. We wrote the NVMe > spec in such a way that vendors must provide single-sector-atomicity > guarantees, and I hope they haven't managed to wiggle some nonsense > into the spec that allows them to not make that guarantee. The below > is a quote from the 1.4 spec. For those not versed in NVMe spec-ese, > "0's based value" means that putting a zero in this field means the > value of AWUPF is 1. Wow - that's the first time I've seen someone go on the record as saying a sector write is atomic (albeit only for NVMe disks) without having it instantly debated! Sadly there's no way of guaranteeing this atomicity from userspace if https://youtu.be/-oP2BOsMpdo?t=3557 (where Chris Mason(?) warns there can be corner cases trying to use O_DIRECT) is to be believed though? > I take neither blame nor credit for what other storage standards may > implement; this is the only one I had a hand in, and I had to fight > hard to get it. So there's no consensus for SATA/SCSI etc (https://stackoverflow.com/questions/2009063/are-disk-sector-writes-atomic )? Just need to wait until there's NVMe everywhere :-) -- Sitsofe | http://sucs.org/~sits/
