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I'm creating sparse files in python as follows:
>>> f = open('testfile', 'ab')
>>> f.truncate(1024000)
>>> f.close()
when the file is done, it takes up 0 disk space, but its inode size is set to my truncated value (1000K):
igor47@piglet:~/test$ ls -lh testfile
-rw-r--r-- 1 igor47 igor47 1000K 2010-07-09 04:02 testfile
igor47@piglet:~/test$ du -hs testfile
0 testfile
How do I get the file's real space usage (allocated size) inside python? The stat call returns the file's apparent size, and I have no idea how else to get the real usage other than to read the entire file (it may become quite large)
>>> os.stat('testfile').st_size
1024000
803
In computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when the file itself is partially empty.
Sparse Files are a type of computer file that allows for efficient storage allocation for large data. A file is considered to be sparse when much of its data is zero (empty data). Support for the creation of such files is generally provided by the File system.
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.
>>> os.stat('testfile').st_blocks*512
0
Tadaa :)
st_blocks is the number of 512-byte blocks actually allocated to the file. Note that st_blocks is not guaranteed to be present in all operating systems, but those that support sparse files generally do.
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