On Wed, Mar 27, 2024 at 01:37:41PM +0000, John Garry wrote: > On 27/03/2024 03:50, 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, > > We want to support untorn writes for bdev file operations - how can we set > the inode block size there? Currently it is based on logical block size. ioctl(BLKBSZSET), I guess? That currently limits to PAGE_SIZE, but I think we can remove that limitation with the bs>PS patches. > > and then all writes to > > that inode were guaranteed to be untorn. This would also be simpler to > > implement for buffered writes. > > We did consider that. Won't that lead to the possibility of breaking > existing applications which want to do regular unaligned writes to these > files? We do know that mysql/innodb does have some "compressed" mode of > operation, which involves regular writes to the same file which wants untorn > writes. If you're talking about "regular unaligned buffered writes", then that won't break. If you cross a folio boundary, the result may be torn, but if you're crossing a block boundary you expect that. > Furthermore, untorn writes in HW are expensive - for SCSI anyway. Do we > always want these for such a file? Do untorn writes actually exist in SCSI? I was under the impression nobody had actually implemented them in SCSI hardware. > We saw untorn writes as not being a property of the file or even the inode > itself, but rather an attribute of the specific IO being issued from the > userspace application. The problem is that keeping track of that is expensive for buffered writes. It's a model that only works for direct IO. Arguably we could make it work for O_SYNC buffered IO, but that'll require some surgery. > > Who's asking for this more complex interface? > > It's not a case of someone specifically asking for this interface. This is > just a proposal to satisfy userspace requirement to do untorn writes in a > generic way. > > From a user point-of-view, untorn writes for a regular file can be enabled > for up to a specific size* with FS_IOC_SETFLAGS API. Then they need to > follow alignment and size rules for issuing untorn writes, but they would > always need to do this. In addition, the user may still issue regular > (tearable) writes to the file. > > * I think that we could change this to only allow writes for that specific > size, which was my proposal for buffered IO. > > Thanks, > John >
