The number of blocks allocated to a sparse file

Question

Is there any way to access the number of blocks allocated to a file with the standard Java File API? Or even do it with some unsupported & undocumented API underneat. Anything to avoid native code plugins.

I'm talking about the st_blocks field of struct stat that the fstat/stat syscalls work on in Unix.

What I want to do is to create a sparse copy of a file that now has lots of redundant data, i.e. make a new copy of it, only containing the active data but sparsely written to it. Then swap the two files with an atomic rename/link operation. But I need a way to find out how many blocks are allocated to the file beforehand, it might already have been sparsely copied. The old file is then removed.

This will be used to free up disk space in a database application that is 100% Java. The benefit on relying on sparse file support in the filesystem is that I would not have to change the index that point out the location where the data is, that increases the complexity of the task at hand.

I think I can do somewhat well by relying on the file timestamp to see if files have already been cleaned up. But this intrigued me. I can not even find anything in the java 7 NIO.2 API for file attribute access at this level.

Answer 1

The only way I can think of is to use ls -s filename to get the actual size of the file on disk. http://www.lrdev.com/lr/unix/sparsefile.html