diff options
Diffstat (limited to 'manual')
| -rw-r--r-- | manual/filesys.texi | 94 |
1 files changed, 94 insertions, 0 deletions
diff --git a/manual/filesys.texi b/manual/filesys.texi index 7d55b43cf2..0f2e3dc3be 100644 --- a/manual/filesys.texi +++ b/manual/filesys.texi @@ -1723,6 +1723,7 @@ modify the attributes of a file. access a file. * File Times:: About the time attributes of a file. * File Size:: Manually changing the size of a file. +* Storage Allocation:: Allocate backing storage for files. @end menu @node Attribute Meanings @@ -3233,6 +3234,99 @@ is a requirement of @code{mmap}. The program has to keep track of the real size, and when it has finished a final @code{ftruncate} call should set the real size of the file. +@node Storage Allocation +@subsection Storage Allocation +@cindex allocating file storage +@cindex file allocation +@cindex storage allocating + +@cindex file fragmentation +@cindex fragmentation of files +@cindex sparse files +@cindex files, sparse +Most file systems support allocating large files in a non-contiguous +fashion: the file is split into @emph{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 @emph{sparse files} with @emph{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 @code{mmap} can still +result in @code{SIGBUS}. + +@deftypefun int posix_fallocate (int @var{fd}, off_t @var{offset}, off_t @var{length}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +@c If the file system does not support allocation, +@c @code{posix_fallocate} has a race with file extension (if +@c @var{length} is zero) or with concurrent writes of non-NUL bytes (if +@c @var{length} is positive). + +Allocate backing store for the region of @var{length} bytes starting at +byte @var{offset} in the file for the descriptor @var{fd}. The file +length is increased to @samp{@var{length} + @var{offset}} if necessary. + +@var{fd} must be a regular file opened for writing, or @code{EBADF} is +returned. If there is insufficient disk space to fulfill the allocation +request, @code{ENOSPC} is returned. + +@strong{Note:} If @code{fallocate} is not available (because the file +system does not support it), @code{posix_fallocate} is emulated, which +has the following drawbacks: + +@itemize @bullet +@item +It is very inefficient because all file system blocks in the requested +range need to be examined (even if they have been allocated before) and +potentially rewritten. In contrast, with proper @code{fallocate} +support (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). + +@item +There is a race condition if another thread or process modifies the +underlying file in the to-be-allocated area. Non-null bytes could be +overwritten with null bytes. + +@item +If @var{fd} has been opened with the @code{O_APPEND} flag, the function +will fail with an @code{errno} value of @code{EBADF}. + +@item +If @var{length} is zero, @code{ftruncate} is used to increase the file +size as requested, without allocating file system blocks. There is a +race condition which means that @code{ftruncate} can accidentally +truncate the file if it has been extended concurrently. +@end itemize + +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 @code{fallocate} function, with a +zero flag argument. For the @code{fallocate} function, @theglibc{} does +not perform allocation emulation if the file system does not support +allocation. Instead, an @code{EOPNOTSUPP} is returned to the caller. + +@end deftypefun + +@deftypefun int posix_fallocate64 (int @var{fd}, off64_t @var{length}, off64_t @var{offset}) +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} + +This function is a variant of @code{posix_fallocate64} which accepts +64-bit file offsets on all platforms. + +@end deftypefun + @node Making Special Files @section Making Special Files @cindex creating special files |