There is no way for any backup program to tell the difference between a normal and sparse file since (as with all Unix flavors) a regular file's format and contents are determined solely by the creating program. What frecover and Omniback will do is to see a string of nulls, stop copying them to disk until non-zero data is encountered and perform an lseek to skip to that particular record and continue writing. The filesystem remembers the empty space by not assigning inodes that to the data areas not written.
It is very important that 1723142 is NOT the true size of the file. If you use cp or tar or cpio or pax or dump, they will all create a copy that is 4501585 in size. This is the true size of the file and if this is a database file, the unoccupied space may eventually become occupied, eventually increasing the orginal file's size. It is very important to note that the file behaves exactly the same way whether the null records exist or not. To see this, use the cksum command on both files. You can also use the cmp command to compare the files. Other than the result from du, the files are truly identical.
It is very seldom the case that a backup then restore of a set files and directories will be the same size. A sparse file can make the copied result larger, but a sparse directory (a directory that once had thousands of files but they were deleted) when copied will be much smaller. This is normal behavior.
As far as symlinks, this can be quite complicated to resolve. A symlink is really just a string that contains the link. There is nothing truly connecting the symlink to a real file or path. So if you restore the link on another system, it may point to nothing. A hard link is just a directory entry that points to the same file and will always work when restored.
Bill Hassell, sysadmin
