The xfs_fsr manpage contains the following notes:
| xfs_fsr improves the layout of extents for each file by copying the
| entire file to a temporary location and then interchanging the data
| extents of the target and temporary files in an atomic manner. This
| method requires that enough free disk space be available to copy any
| given file and that the space be less fragmented then the original
| file. It also requires the owner of the file to have enough
| remaining filespace quota to do the copy on systems running quotas.
| xfs_fsr generates a warning message if space is not suffi- cient to
| improve the target file.
Is it safe to run xfs_fsr on a file which is regularly updated? It
seems that if a copy is made and later linked as the original, updates
to the file might be lost. Is this really the case?