Estimating the volume of after-image extent size is not necessary when you are using variable extents. The amount of after-image data that is written to the extents is no different when using variable extents as opposed to fixed extents. When you are using variable extents, the following factors should be taken into account:
No limits — The size of the variable length extent is limited only to the size of the file system (provided large files are enabled on your system).
Fills to largest size — The variable extents will fill to the largest size available for the files.
Easier after-image management — Not having to deal with multiple extents becoming full and not having to change extents and do extent management more often can save work.
No sizing rules — Since the after-image extents are not pre-allocated and pre-formatted, the guidelines for creating extents do not apply.
No limits — The size of the variable length extent is limited only to the size of the file system (provided large files are enabled on your system).