Most file systems support allocating large files in a non-contiguous fashion: the file is split into fragments which are allocated sequentially, but the fragments themselves can be scattered across the disk. File systems generally try to avoid such fragmentation because it decreases performance, but if a file gradually increases in size, there might be no other option than to fragment it. In addition, many file systems support sparse files with holes: regions of null bytes for which no backing storage has been allocated by the file system. When the holes are finally overwritten with data, fragmentation can occur as well.
Explicit allocation of storage for yet-unwritten parts of the file can
help the system to avoid fragmentation. Additionally, if storage
pre-allocation fails, it is possible to report the out-of-disk error
early, often without filling up the entire disk. However, due to
deduplication, copy-on-write semantics, and file compression, such
pre-allocation may not reliably prevent the out-of-disk-space error from
occurring later. Checking for write errors is still required, and
writes to memory-mapped regions created with
mmap can still
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
Allocate backing store for the region of length bytes starting at byte offset in the file for the descriptor fd. The file length is increased to ‘length + offset’ if necessary.
fd must be a regular file opened for writing, or
returned. If there is insufficient disk space to fulfill the allocation
ENOSPC is returned.
fallocate is not available (because the file
system does not support it),
posix_fallocate is emulated, which
has the following drawbacks:
fallocatesupport (see below), the file system can examine the internal file allocation data structures and eliminate holes directly, maybe even using unwritten extents (which are pre-allocated but uninitialized on disk).
O_WRONLYflag, the function will fail with an
O_APPENDflag, the function will fail with an
ftruncateis used to increase the file size as requested, without allocating file system blocks. There is a race condition which means that
ftruncatecan accidentally truncate the file if it has been extended concurrently.
On Linux, if an application does not benefit from emulation or if the
emulation is harmful due to its inherent race conditions, the
application can use the Linux-specific
fallocate function, with a
zero flag argument. For the
fallocate function, the GNU C Library does
not perform allocation emulation if the file system does not support
allocation. Instead, an
EOPNOTSUPP is returned to the caller.