On 09/03/2025 21:51, Dave Chinner wrote:
Mon, Mar 03, 2025 at 05:11:13PM +0000, John Garry wrote:
Currently atomic write support requires dedicated HW support. 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
