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This imposes >> a restriction on the filesystem that disk blocks need to be aligned and >> contiguously mapped to FS blocks to issue atomic writes. >> >> XFS has no method to guarantee FS block alignment for regular, >> non-RT files. As such, atomic writes are currently limited to 1x FS block >> there. >> >> To deal with the scenario that we are issuing an atomic write over >> misaligned or discontiguous data blocks - and raise the atomic write size >> limit - support a SW-based software emulated atomic write mode. For XFS, >> this SW-based atomic writes would use CoW support to issue emulated untorn >> writes. >> >> It is the responsibility of the FS to detect discontiguous atomic writes >> and switch to IOMAP_DIO_ATOMIC_SW mode and retry the write. Indeed, >> SW-based atomic writes could be used always when the mounted bdev does >> not support HW offload, but this strategy is not initially expected to be >> used. > So now seeing how these are are to be used, these aren't "hardware" > and "software" atomic IOs. They are block layer vs filesystem atomic > IOs. > > We can do atomic IOs in software in the block layer drivers (think > loop or dm-thinp) rather than off-loading to storage hardware. > > Hence I think these really need to be named after the layer that > will provide the atomic IO guarantees, because "hw" and "sw" as they > are currently used are not correct. e.g something like > IOMAP_FS_ATOMIC and IOMAP_BDEV_ATOMIC which indicates which layer > should be providing the atomic IO constraints and guarantees. I'd prefer IOMAP_REQ_ATOMIC instead (of IOMAP_BDEV_ATOMIC), as we are using REQ_ATOMIC for those BIOs only. Anything which the block layer and below does with REQ_ATOMIC is its business, as long as it guarantees atomic submission. But I am not overly keen on that name, as it clashes with block layer names (naturally). And IOMAP_FS_ATOMIC seems a bit vague, but I can't think of anything else. Darrick, any opinion on this? Cheers, John