sparse file

Sparse files are a result of the way Unix file systems allocate storage space. Sparse files appear to take up much more disk space than they actually do.

Traditional Unix file systems have thirteen disk block pointers in the inode. The first 10 are direct pointers; that is they point to actual disk blocks.

The next pointer is an indirect pointer: it points to a block that contains pointers to actual storage blocks. If the pointers are 32 bit numbers (more likely to be larger nowadays but this makes the math easy for us), there will be 128 blocks covered by the 11th pointer. The next points to double indirect blocks: blocks which themselves contain more pointers to the actual blocks. The 13th is triple indirect: pointers to blocks of pointers to blocks of pointers. This scheme lets you have very large files without very large inode records.

If a program opens a new file and seeks to a point beyond that covered by the first 10 pointers and writes data there, the OS doesn't bother to allocate data blocks that are not used, so those first 10 pointers are still empty. The same is true for some of the indirect pointers if the next seek extends beyond their reach. Database files using hash keys often create such files as a natural result of the hash key creation. The file may show it has a very large size in an ls -l listing, but it could actually be consuming only a very few disk blocks. If you read data from such a file, the OS just returns null bytes for the unallocated space, which means that naively copying it will NOT create another sparse file (although GNU cp is aware of sparse files and will preserve the sparseness). The same is true when backing up to tape (again, modern utilities can properly handle this).

See Backups and sparse files and Sparse files and hashing also.

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