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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.
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The EXT2 file system divides the logical partition that it occupies into Block Groups. Each group duplicates information critical to the integrity of the file system as well as holding real files and directories as blocks of information and data.
That is why sparse files originally take up less space than their real size is. This file type is supported by most file systems: BTRFS, NILFS, ZFS, NTFS[2], ext2, ext3, ext4, XFS, JFS, ReiserFS, Reiser4, UFS, Rock Ridge, UDF, ReFS, APFS, F2FS.
A file format that saves storage space by recording only actual data. Whereas regular files record empty fields as blank data or runs of nulls, a sparse file includes meta-data that describe where the runs of non-data are located. The reported file size is always the size of the entire file.
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.
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