Re: [PATCH v2] xfs_io: support a basic extent swap command

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 08, 2018 at 10:57:19AM -0500, Brian Foster wrote:
> Extent swap is a low level mechanism exported by XFS to facilitate
> filesystem defragmentation. It is typically invoked by xfs_fsr under
> conditions that will atomically adjust inode extent state without
> loss of file data.
> 
> While xfs_fsr provides some debug capability to tailor its behavior,
> it is not flexible enough to facilitate low level tests of the
> extent swap mechanism. For example, xfs_fsr may skip swaps between
> inodes that consist solely of preallocated extents because it
> considers such files already 100% defragmented. Further, xfs_fsr
> copies data between files where doing so may be unnecessary and thus
> inefficient for lower level tests.
> 
> Add a basic swapext command to xfs_io that allows userspace
> invocation of the command under more controlled conditions. This
> facilites targeted tests without interference from xfs_fsr policy,
> such as using files with only preallocated extents, known/expected
> failure cases, etc. This command makes no effort to retain data
> across the operation. As such, it is for testing purposes only.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D