On Thu, Mar 28, 2024 at 07:31:45AM +1100, Dave Chinner wrote: > On Wed, Mar 27, 2024 at 03:50:07AM +0000, Matthew Wilcox wrote: > > On Tue, Mar 26, 2024 at 01:38:03PM +0000, John Garry wrote: > > > The goal here is to provide an interface that allows applications use > > > application-specific block sizes larger than logical block size > > > reported by the storage device or larger than filesystem block size as > > > reported by stat(). > > > > > > With this new interface, application blocks will never be torn or > > > fractured when written. For a power fail, for each individual application > > > block, all or none of the data to be written. A racing atomic write and > > > read will mean that the read sees all the old data or all the new data, > > > but never a mix of old and new. > > > > > > Three new fields are added to struct statx - atomic_write_unit_min, > > > atomic_write_unit_max, and atomic_write_segments_max. For each atomic > > > individual write, the total length of a write must be a between > > > atomic_write_unit_min and atomic_write_unit_max, inclusive, and a > > > power-of-2. The write must also be at a natural offset in the file > > > wrt the write length. For pwritev2, iovcnt is limited by > > > atomic_write_segments_max. > > > > > > There has been some discussion on supporting buffered IO and whether the > > > API is suitable, like: > > > https://lore.kernel.org/linux-nvme/ZeembVG-ygFal6Eb@xxxxxxxxxxxxxxxxxxxx/ > > > > > > Specifically the concern is that supporting a range of sizes of atomic IO > > > in the pagecache is complex to support. For this, my idea is that FSes can > > > fix atomic_write_unit_min and atomic_write_unit_max at the same size, the > > > extent alignment size, which should be easier to support. We may need to > > > implement O_ATOMIC to avoid mixing atomic and non-atomic IOs for this. I > > > have no proposed solution for atomic write buffered IO for bdev file > > > operations, but I know of no requirement for this. > > > > The thing is that there's no requirement for an interface as complex as > > the one you're proposing here. I've talked to a few database people > > and all they want is to increase the untorn write boundary from "one > > disc block" to one database block, typically 8kB or 16kB. > > > > So they would be quite happy with a much simpler interface where they > > set the inode block size at inode creation time, and then all writes to > > that inode were guaranteed to be untorn. This would also be simpler to > > implement for buffered writes. > > You're conflating filesystem functionality that applications will use > with hardware and block-layer enablement that filesystems and > filesystem utilities need to configure the filesystem in ways that > allow users to make use of atomic write capability of the hardware. > > The block layer functionality needs to export everything that the > hardware can do and filesystems will make use of. The actual > application usage and setup of atomic writes at the filesystem/page > cache layer is a separate problem. i.e. The block layer interfaces > need only support direct IO and expose limits for issuing atomic > direct IO, and nothing more. All the more complex stuff to make it > "easy to use" is filesystem level functionality and completely > outside the scope of this patchset.... A CoW filesystem can implement atomic writes without any block device support. It seems to me that might have been the easier place to start - start by getting the APIs right, then do all the plumbing for efficient untorn writes on non CoW filesystems...
