On Tue, Feb 28, 2017 at 4:57 PM, Christoph Hellwig <hch@xxxxxx> wrote: > Hi all, > > this series implements a new O_ATOMIC flag for failure atomic writes > to files. It is based on and tries to unify to earlier proposals, > the first one for block devices by Chris Mason: > > https://lwn.net/Articles/573092/ > > and the second one for regular files, published by HP Research at > Usenix FAST 2015: > > https://www.usenix.org/conference/fast15/technical-sessions/presentation/verma > > It adds a new O_ATOMIC flag for open, which requests writes to be > failure-atomic, that is either the whole write makes it to persistent > storage, or none of it, even in case of power of other failures. > > There are two implementation various of this: on block devices O_ATOMIC > must be combined with O_(D)SYNC so that storage devices that can handle > large writes atomically can simply do that without any additional work. > This case is supported by NVMe. > > The second case is for file systems, where we simply write new blocks > out of places and then remap them into the file atomically on either > completion of an O_(D)SYNC write or when fsync is called explicitly. > > The semantics of the latter case are explained in detail at the Usenix > paper above. > > Last but not least a new fcntl is implemented to provide information > about I/O restrictions such as alignment requirements and the maximum > atomic write size. > > The implementation is simple and clean, but I'm rather unhappy about > the interface as it has too many failure modes to bullet proof. For > one old kernels ignore unknown open flags silently, so applications > have to check the F_IOINFO fcntl before, which is a bit of a killer. > Because of that I've also not implemented any other validity checks > yet, as they might make thing even worse when an open on a not supported > file system or device fails, but not on an old kernel. Maybe we need > a new open version that checks arguments properly first? > [CC += linux-api@xxxxxxxxxxxxxxx] for that question and for the new API > Also I'm really worried about the NVMe failure modes - devices simply > advertise an atomic write size, with no way for the device to know > that the host requested a given write to be atomic, and thus no > error reporting. This is made worse by NVMe 1.2 adding per-namespace > atomic I/O parameters that devices can use to introduce additional > odd alignment quirks - while there is some language in the spec > requiring them not to weaken the per-controller guarantees it all > looks rather weak and I'm not too confident in all implementations > getting everything right. > > Last but not least this depends on a few XFS patches, so to actually > apply / run the patches please use this git tree: > > git://git.infradead.org/users/hch/vfs.git O_ATOMIC > > Gitweb: > > http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/O_ATOMIC
