Re: [PATCH v2 00/16] block atomic writes

On Wed, Jan 10, 2024 at 10:19:29AM +0100, Christoph Hellwig wrote:

On Wed, Jan 10, 2024 at 10:04:00AM +1100, Dave Chinner wrote:

quoted

Hence history teaches us that we should be designing the API around
the generic filesystem function required (hard alignment of physical
extent allocation), not the specific use case that requires that
functionality.

I disagree.  The alignment requirement is an artefact of how you
implement atomic writes.  As the fs user I care that I can do atomic
writes on a file and need to query how big the writes can be and
what alignment is required.

The forcealign feature is a sensible fs side implementation of that
if using hardware based atomic writes with alignment requirements,
but it is a really lousy userspace API.

So with John's API proposal for XFS with hardware alignment based atomic
writes we could still use force align.

Requesting atomic writes for an inode will set the forcealign flag
and the extent size hint, and after that it'll report atomic write
capabilities.  Roughly the same implementation, but not an API
tied to an implementation detail.

Sounds good to me!  So to summarize, this is approximately what
userspace programs would have to do something like this:

struct statx statx;
struct fsxattr fsxattr;
int fd = open('/foofile', O_RDWR | O_DIRECT);

ioctl(fd, FS_IOC_GETXATTR, &fsxattr);

fsxattr.fsx_xflags |= FS_XFLAG_FORCEALIGN | FS_XFLAG_WRITE_ATOMIC;
fsxattr.fsx_extsize = 16384; /* only for hardware no-tears writes */

ioctl(fd, FS_IOC_SETXATTR, &fsxattr);

statx(fd, "", AT_EMPTY_PATH, STATX_ALL | STATX_WRITE_ATOMIC, &statx);

if (statx.stx_atomic_write_unit_max >= 16384) {
	pwrite(fd, &iov, 1, 0, RWF_SYNC | RWF_ATOMIC);
	printf("HAPPY DANCE\n");
}

(Assume we bail out on errors.)

--D