holes.md

Motivation

Virtual machine or ISO disc images or other "file systems within a file" or sometimes object / database files can have intentionally large holes / sparsity. Corrupted torrent downloads may, meanwhile, have very large holes by accident.

While these files can be easily identified with ls -ls or stat comparing the allocated blocks and seek-addressable file size, I could find no standard Unix command-line tool to count/list holes. (A quick web search will show numerous C programming examples to use the Unix API to list holes, though.) So here is (probably another) one.

Usage

  holes [optional-params] [files: string...]

Show hole & data segments for files

  -f=, --format= string "" emit format interpolating:
                             $count : number of data|hole segments
                             $path  : path name of REGULAR FILE from $*
                             $map   : map of all data&hole segments
                             $nul   : a NUL byte
                           "" => "$count\t$path\n$holes\n"

Example

truncate -s 5000 x; printf hi >>x; holes x

prints on a file system with 4096-byte blocks:

2 x
	hole	4096
	data	906

Related

filefrag is a similar but distinct utility which uses a less portable Linux FIEMAP ioctl. Distinctness-wise, for example, I get "4 extents" from a filefrag foo.xfs, while holes reports 42814 hole|data segments¹.

Footnotes

1. This is admittedly after running xfs_fsr. ↩