How to remove some blocks from a sparse file on a ext2/ext3 filesystem

Question

The ext2/ext3 filesystem automatically allocate blocks when you write a sparse file, but when I no longer want some blocks of them, I found no ways to do it. It feels like using malloc() without free(). Is it possible to "free" some blocks of a sparse file? If it is, how? Don't tell me to copy it to a new file. It's too boring and needs a lot of disk space.

Answer 1

Since Linux 2.6.38, there is a flag to fallocate called FALLOC_FL_PUNCH_HOLE which should do what you want, i.e. deallocate file space at arbitrary locations.

fallocate(fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE, offset, len);

will punch a hole into the file indicated by descriptor fd. The hole will start at offset and have length len, both measured in bytes. Only whole blocks will actually be removed, partial blocks will be zeroed out instead.