|System Calls Manual
fallocate - manipulate file space
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h>
int fallocate(int fd, int mode, off_t offset, off_t len);
This is a nonportable, Linux-specific system call. For the portable, POSIX.1-specified method of ensuring that space is allocated for a file, see posix_fallocate(3).
fallocate() allows the caller to directly manipulate the allocated disk space for the file referred to by fd for the byte range starting at offset and continuing for len bytes.
The mode argument determines the operation to be performed on the given range. Details of the supported operations are given in the subsections below.
Allocating disk space¶
The default operation (i.e., mode is zero) of fallocate() allocates the disk space within the range specified by offset and len. The file size (as reported by stat(2)) will be changed if offset+len is 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.
After a successful call, subsequent writes into the range specified by offset and len are guaranteed not to fail because of lack of disk space.
If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the behavior of the call is similar, but the file size will not be changed even if offset+len is greater than the file size. Preallocating zeroed blocks beyond the end of the file in this manner is useful for optimizing append workloads.
If the FALLOC_FL_UNSHARE_RANGE flag is specified in mode, shared file data extents will be made private to the file to guarantee that a subsequent write will not fail due to lack of space. Typically, this will be done by performing a copy-on-write operation on all shared data in the file. This flag may not be supported by all filesystems.
Because allocation is done in block size chunks, fallocate() may allocate a larger range of disk space than was specified.
Deallocating file space¶
Specifying the 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 zeros.
The FALLOC_FL_PUNCH_HOLE flag must be ORed with FALLOC_FL_KEEP_SIZE in mode; in other words, even when punching off the end of the file, the file size (as reported by stat(2)) does not change.
Not all filesystems support FALLOC_FL_PUNCH_HOLE; if a filesystem doesn't support the operation, an error is returned. The operation is supported on at least the following filesystems:
Collapsing file space¶
Specifying the 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 for 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.
A filesystem may place limitations on the granularity of the operation, in order to ensure efficient implementation. Typically, offset and len must be a multiple of the filesystem logical block size, which varies according to the filesystem type and configuration. If a filesystem has such a requirement, fallocate() fails with the error EINVAL if this requirement is violated.
If the region specified by offset plus len reaches or passes the end of file, an error is returned; instead, use ftruncate(2) to truncate a file.
No other flags may be specified in mode in conjunction with FALLOC_FL_COLLAPSE_RANGE.
As at Linux 3.15, FALLOC_FL_COLLAPSE_RANGE is supported by ext4 (only for extent-based files) and XFS.
Zeroing file space¶
Specifying the FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15) in mode zeros space in the byte range starting at offset and continuing for 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 zeros.
Zeroing is done within the filesystem preferably by converting the range into unwritten extents. This approach means that the specified range will not be physically zeroed out on the device (except for partial blocks at the either end of the range), and I/O is (otherwise) required only to update metadata.
If the FALLOC_FL_KEEP_SIZE flag is additionally specified in mode, the behavior of the call is similar, but the file size will not be changed even if offset+len is greater than the file size. This behavior is the same as when preallocating space with FALLOC_FL_KEEP_SIZE specified.
Not all filesystems support FALLOC_FL_ZERO_RANGE; if a filesystem doesn't support the operation, an error is returned. The operation is supported on at least the following filesystems:
- XFS (since Linux 3.15)
- ext4, for extent-based files (since Linux 3.15)
- SMB3 (since Linux 3.17)
- Btrfs (since Linux 4.16)
Increasing file space¶
Specifying the 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 at 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 len bytes.
This mode has the same limitations as FALLOC_FL_COLLAPSE_RANGE regarding the granularity of the operation. If the granularity requirements are not met, fallocate() fails with the error EINVAL. If the offset is equal to or greater than the end of file, an error is returned. For such operations (i.e., inserting a hole at the end of file), ftruncate(2) should be used.
No other flags may be specified in mode in conjunction with FALLOC_FL_INSERT_RANGE.
FALLOC_FL_INSERT_RANGE requires filesystem support. Filesystems that support this operation include XFS (since Linux 4.1) and ext4 (since Linux 4.2).
On success, fallocate() returns zero. On error, -1 is returned and errno is set to indicate the error.
- fd is not a valid file descriptor, or is not opened for writing.
- offset+len exceeds the maximum file size.
- mode is FALLOC_FL_INSERT_RANGE, and the current file size+len exceeds the maximum file size.
- A signal was caught during execution; see signal(7).
- offset was less than 0, or len was less than or equal to 0.
- mode is FALLOC_FL_COLLAPSE_RANGE and the range specified by offset plus len reaches or passes the end of the file.
- mode is FALLOC_FL_INSERT_RANGE and the range specified by offset reaches or passes the end of the file.
- mode is FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but either offset or len is not a multiple of the filesystem block size.
- mode contains one of FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE and also other flags; no other flags are permitted with FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE.
- mode is FALLOC_FL_COLLAPSE_RANGE, FALLOC_FL_ZERO_RANGE, or FALLOC_FL_INSERT_RANGE, but the file referred to by fd is not a regular file.
- An I/O error occurred while reading from or writing to a filesystem.
- fd does not refer to a regular file or a directory. (If fd is a pipe or FIFO, a different error results.)
- There is not enough space left on the device containing the file referred to by fd.
- This kernel does not implement fallocate().
- The filesystem containing the file referred to by fd does not support this operation; or the mode is not supported by the filesystem containing the file referred to by fd.
- The file referred to by fd is marked immutable (see chattr(1)).
- mode specifies FALLOC_FL_PUNCH_HOLE, FALLOC_FL_COLLAPSE_RANGE, or FALLOC_FL_INSERT_RANGE and the file referred to by fd is marked append-only (see chattr(1)).
- The operation was prevented by a file seal; see fcntl(2).
- fd refers to a pipe or FIFO.
- mode specifies FALLOC_FL_COLLAPSE_RANGE or FALLOC_FL_INSERT_RANGE, but the file referred to by fd is currently being executed.
fallocate() is available since Linux 2.6.23. Support is provided since glibc 2.10. The FALLOC_FL_* flags are defined in glibc headers only since glibc 2.18.
fallocate() is Linux-specific.
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