They can all change file size according to my test. why can they all change file to larger and to shorter? what's the difference between fallocate and ftruncate?
ftruncate is a simple, single-purpose function. Per the POSIX documentation, it simply sets the file to the requested length:
fildes refers to a regular file, the
ftruncate() function shall cause the size of the file to be truncated to
ftruncate() is also a standard POSIX function and is portable. Note that POSIX does not specify <em>how</em> an OS sets the file length, such as whether or not a file set to any length is a sparse file.
fallocate() is a Linux-specific function that does a lot more, and in very specific ways:<blockquote>
Allocating disk space
The default operation (i.e., mode is zero) of fallocate() allocates
the disk space within the range specified by
file size (as reported by
stat(2)) will be changed if
greater than the file size. Any subregion within the range specified
by offset and len that did not contain data before the call will be
initialized to zero. This default behavior closely resembles the
behavior of the
posix_fallocate(3) library function, and is intended
as a method of optimally implementing that function.
Deallocating file space
FALLOC_FL_PUNCH_HOLE flag (available since Linux
2.6.38) in mode deallocates space (i.e., creates a hole) in the byte
range starting at
offset and continuing for
len bytes. Within the
specified range, partial filesystem blocks are zeroed, and whole
filesystem blocks are removed from the file. After a successful
call, subsequent reads from this range will return zeroes.
Collapsing file space
FALLOC_FL_COLLAPSE_RANGE flag (available since Linux
3.15) in mode removes a byte range from a file, without leaving a
hole. The byte range to be collapsed starts at
offset and continues
len bytes. At the completion of the operation, the contents of
the file starting at the location
offset+len will be appended at the
location offset, and the file will be
len bytes smaller.
Zeroing file space
FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15)
in mode zeroes space in the byte range starting at
len bytes. Within the specified range, blocks are
preallocated for the regions that span the holes in the file. After
a successful call, subsequent reads from this range will return
Increasing file space
FALLOC_FL_INSERT_RANGE flag (available since Linux
4.1) in mode increases the file space by inserting a hole within the
file size without overwriting any existing data. The hole will start
offset and continue for
len bytes. When inserting the hole inside
file, the contents of the file starting at
offset will be shifted
upward (i.e., to a higher file offset) by
len bytes. Inserting a
hole inside a file increases the file size by
fallocate is used to preallocate blocks to a file
The following command will allocate a file with a size of 1GB.
fallocate -l 1G test_file1.img
ftruncate - set a file to a specified length
As I now know:
1.fallocate can't change file to shorter. it add actual space to file.
2.ftruncate add len to "describe", just like declaration.
With ftruncate you tell linux the size you want this file to be. It will truncate extra space if the file is getting shorter (including freeing up disk space) or add zeros, allocating disk space if you make the file longer.
fallocate is a general purpose function to effect change to a range to bytes belonging to a file.
Depending on how you use fallocate, you can accomplish everything you could with ftruncate with fallocate. Its just a little more complicated as you will have to know which range you need to allocate/deallocate (initial offset+length).
With fallocate you can pre allocate disk space without logically growing a file (example a zero byte file at the ls level that uses 1GB).
I just wrote a C program to perform high performance gzipping of a file with direct I/O using fallocate and ftruncate. I use fallocate to pre-allocate 64MB at a time to the file. In the end I use ftruncate to trim the excess space allocated.
Works perfectly with XFS, I confirmed ftruncate actually frees disk space with xfs_bmap -vp on a few files.