In some cases it may be desirable to change the geometry of the EC parity mirror on a file. This would be useful if "hot" files have a narrow raidset to reduce overhead during reconstruction (e.g. 4+2) and "cold" files have a wide raidset (e.g. 32+3) to reduce overhead but still maintain resiliency.

It should be possible to do this without first deleting the existing parity mirror and creating a new one, since that would leave the file unprotected until the new parity mirror is attached. It seems reasonable to allow something like lfs mirror extend --ec 32+3 --replace FILE (or similar) to create a victim (volatile) file that contains only the new EC parity mirror component (with suitable OST selection taking into account the 32 OSTs the data stripes are using) and then atomically swap the new EC parity mirror with the existing parity mirror on FILE and the old parity mirror is deleted with the victim file.