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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | USE CASES | NOTES | SEE ALSO | COLOPHON |
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IOCTL-XFS-COMMIT-RANGE(2) System Calls Manual IOCTL-XFS-COMMIT-RANGE(2)
ioctl_xfs_start_commit - prepare to exchange the contents of two
files ioctl_xfs_commit_range - conditionally exchange the contents
of parts of two files
#include <sys/ioctl.h>
#include <xfs/xfs_fs.h>
int ioctl(int file2_fd, XFS_IOC_START_COMMIT, struct
xfs_commit_range *arg);
int ioctl(int file2_fd, XFS_IOC_COMMIT_RANGE, struct
xfs_commit_range *arg);
Given a range of bytes in a first file file1_fd and a second range
of bytes in a second file file2_fd, this ioctl(2) exchanges the
contents of the two ranges if file2_fd passes certain freshness
criteria.
Before exchanging the contents, the program must call the
XFS_IOC_START_COMMIT ioctl to sample freshness data for file2_fd.
If the sampled metadata does not match the file metadata at commit
time, XFS_IOC_COMMIT_RANGE will return EBUSY.
Exchanges are atomic with regards to concurrent file operations.
Implementations must guarantee that readers see either the old
contents or the new contents in their entirety, even if the system
fails.
The system call parameters are conveyed in structures of the
following form:
struct xfs_commit_range {
__s32 file1_fd;
__u32 pad;
__u64 file1_offset;
__u64 file2_offset;
__u64 length;
__u64 flags;
__u64 file2_freshness[5];
};
The field pad must be zero.
The fields file1_fd, file1_offset, and length define the first
range of bytes to be exchanged.
The fields file2_fd, file2_offset, and length define the second
range of bytes to be exchanged.
The field file2_freshness is an opaque field whose contents are
determined by the kernel. These file attributes are used to
confirm that file2_fd has not changed by another thread since the
current thread began staging its own update.
Both files must be from the same filesystem mount. If the two
file descriptors represent the same file, the byte ranges must not
overlap. Most disk-based filesystems require that the starts of
both ranges must be aligned to the file block size. If this is
the case, the ends of the ranges must also be so aligned unless
the XFS_EXCHANGE_RANGE_TO_EOF flag is set.
The field flags control the behavior of the exchange operation.
XFS_EXCHANGE_RANGE_TO_EOF
Ignore the length parameter. All bytes in file1_fd
from file1_offset to EOF are moved to file2_fd, and
file2's size is set to (file2_offset+(file1_length-
file1_offset)). Meanwhile, all bytes in file2 from
file2_offset to EOF are moved to file1 and file1's size
is set to (file1_offset+(file2_length-file2_offset)).
XFS_EXCHANGE_RANGE_DSYNC
Ensure that all modified in-core data in both file
ranges and all metadata updates pertaining to the
exchange operation are flushed to persistent storage
before the call returns. Opening either file
descriptor with O_SYNC or O_DSYNC will have the same
effect.
XFS_EXCHANGE_RANGE_FILE1_WRITTEN
Only exchange sub-ranges of file1_fd that are known to
contain data written by application software. Each
sub-range may be expanded (both upwards and downwards)
to align with the file allocation unit. For files on
the data device, this is one filesystem block. For
files on the realtime device, this is the realtime
extent size. This facility can be used to implement
fast atomic scatter-gather writes of any complexity for
software-defined storage targets if all writes are
aligned to the file allocation unit.
XFS_EXCHANGE_RANGE_DRY_RUN
Check the parameters and the feasibility of the
operation, but do not change anything.
On error, -1 is returned, and errno is set to indicate the error.
Error codes can be one of, but are not limited to, the following:
EBADF file1_fd is not open for reading and writing or is open for
append-only writes; or file2_fd is not open for reading and
writing or is open for append-only writes.
EBUSY The file2 inode number and timestamps supplied do not match
file2_fd.
EINVAL The parameters are not correct for these files. This error
can also appear if either file descriptor represents a
device, FIFO, or socket. Disk filesystems generally
require the offset and length arguments to be aligned to
the fundamental block sizes of both files.
EIO An I/O error occurred.
EISDIR One of the files is a directory.
ENOMEM The kernel was unable to allocate sufficient memory to
perform the operation.
ENOSPC There is not enough free space in the filesystem exchange
the contents safely.
EOPNOTSUPP
The filesystem does not support exchanging bytes between
the two files.
EPERM file1_fd or file2_fd are immutable.
ETXTBSY
One of the files is a swap file.
EUCLEAN
The filesystem is corrupt.
EXDEV file1_fd and file2_fd are not on the same mounted
filesystem.
This API is XFS-specific.
Several use cases are imagined for this system call. Coordination
between multiple threads is performed by the kernel.
The first is a filesystem defragmenter, which copies the contents
of a file into another file and wishes to exchange the space
mappings of the two files, provided that the original file has not
changed.
An example program might look like this:
int fd = open("/some/file", O_RDWR);
int temp_fd = open("/some", O_TMPFILE | O_RDWR);
struct stat sb;
struct xfs_commit_range args = {
.flags = XFS_EXCHANGE_RANGE_TO_EOF,
};
/* gather file2's freshness information */
ioctl(fd, XFS_IOC_START_COMMIT, &args);
fstat(fd, &sb);
/* make a fresh copy of the file with terrible alignment to avoid reflink */
clone_file_range(fd, NULL, temp_fd, NULL, 1, 0);
clone_file_range(fd, NULL, temp_fd, NULL, sb.st_size - 1, 0);
/* commit the entire update */
args.file1_fd = temp_fd;
ret = ioctl(fd, XFS_IOC_COMMIT_RANGE, &args);
if (ret && errno == EBUSY)
printf("file changed while defrag was underway\n");
The second is a data storage program that wants to commit non-
contiguous updates to a file atomically. This program cannot
coordinate updates to the file and therefore relies on the kernel
to reject the COMMIT_RANGE command if the file has been updated by
someone else. This can be done by creating a temporary file,
calling FICLONE(2) to share the contents, and staging the updates
into the temporary file. The FULL_FILES flag is recommended for
this purpose. The temporary file can be deleted or punched out
afterwards.
An example program might look like this:
int fd = open("/some/file", O_RDWR);
int temp_fd = open("/some", O_TMPFILE | O_RDWR);
struct xfs_commit_range args = {
.flags = XFS_EXCHANGE_RANGE_TO_EOF,
};
/* gather file2's freshness information */
ioctl(fd, XFS_IOC_START_COMMIT, &args);
ioctl(temp_fd, FICLONE, fd);
/* append 1MB of records */
lseek(temp_fd, 0, SEEK_END);
write(temp_fd, data1, 1000000);
/* update record index */
pwrite(temp_fd, data1, 600, 98765);
pwrite(temp_fd, data2, 320, 54321);
pwrite(temp_fd, data2, 15, 0);
/* commit the entire update */
args.file1_fd = temp_fd;
ret = ioctl(fd, XFS_IOC_COMMIT_RANGE, &args);
if (ret && errno == EBUSY)
printf("file changed before commit; will roll back\n");
Some filesystems may limit the amount of data or the number of
extents that can be exchanged in a single call.
ioctl(2)
This page is part of the xfsprogs (utilities for XFS filesystems)
project. Information about the project can be found at
⟨http://xfs.org/⟩. If you have a bug report for this manual page,
send it to linux-xfs@vger.kernel.org. This page was obtained from
the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2024-12-02.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
XFS 2024-02-18 IOCTL-XFS-COMMIT-RANGE(2)
Copyright and license for this manual page
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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