sync_file_range — sync a file segment with disk
#define _GNU_SOURCE #include <fcntl.h>
int sync_file_range( |
int fd, |
| off64_t offset, | |
| off64_t nbytes, | |
unsigned int flags); |
sync_file_range() permits
fine control when synchronizing the open file referred to by
the file descriptor fd with disk.
offset is the
starting byte of the file range to be synchronized.
nbytes specifies the
length of the range to be synchronized, in bytes; if
nbytes is zero, then
all bytes from offset
through to the end of file are synchronized. Synchronization
is in units of the system page size: offset is rounded down to a
page boundary; (offset+nbytes-1) is rounded
up to a page boundary.
The flags bit-mask
argument can include any of the following values:
SYNC_FILE_RANGE_WAIT_BEFOREWait upon write-out of all pages in the specified range that have already been submitted to the device driver for write-out before performing any write.
SYNC_FILE_RANGE_WRITEInitiate write-out of all dirty pages in the specified range which are not presently submitted write-out. Note that even this may block if you attempt to write more than request queue size.
SYNC_FILE_RANGE_WAIT_AFTERWait upon write-out of all pages in the range after performing any write.
Specifying flags
as 0 is permitted, as a no-op.
This system call is extremely dangerous and should not
be used in portable programs. None of these operations
writes out the file's metadata. Therefore, unless the
application is strictly performing overwrites of
already-instantiated disk blocks, there are no guarantees
that the data will be available after a crash. There is no
user interface to know if a write is purely an overwrite.
On file systems using copy-on-write semantics (e.g.,
btrfs) an
overwrite of existing allocated blocks is impossible. When
writing into preallocated space, many file systems also
require calls into the block allocator, which this system
call does not sync out to disk. This system call does not
flush disk write caches and thus does not provide any data
integrity on systems with volatile disk write caches.
SYNC_FILE_RANGE_WAIT_BEFORE and
SYNC_FILE_RANGE_WAIT_AFTER
will detect any I/O errors or ENOSPC conditions and will return these
to the caller.
Useful combinations of the flags bits are:
Ensures that all pages in the specified range
which were dirty when sync_file_range() was called are
placed under write-out. This is a
start-write-for-data-integrity operation.
SYNC_FILE_RANGE_WRITEStart write-out of all dirty pages in the specified range which are not presently under write-out. This is an asynchronous flush-to-disk operation. This is not suitable for data integrity operations.
SYNC_FILE_RANGE_WAIT_BEFORE (or
SYNC_FILE_RANGE_WAIT_AFTER)Wait for completion of write-out of all pages in the specified range. This can be used after an earlier SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation to wait for completion of that operation, and obtain its result.
This is a write-for-data-integrity operation that
will ensure that all pages in the specified range
which were dirty when sync_file_range() was called are
committed to disk.
On success, sync_file_range() returns 0; on failure
−1 is returned and errno
is set to indicate the error.
fd is not a
valid file descriptor.
flags
specifies an invalid bit; or offset or nbytes is invalid.
I/O error.
Out of memory.
Out of disk space.
fd refers to
something other than a regular file, a block device, a
directory, or a symbolic link.
This page is part of release 3.25 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright (c) 2006 Andrew Morton <akpmosdl.org> and Copyright 2006 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. 2006-07-05 Initial creation, Michael Kerrisk based on Andrew Morton's comments in fs/sync.c |