IOCTL-XFS-COMMIT-RANGE(2)

NAME
ioctl_xfs_commit_range - conditionally exchange the contents of parts of two files ioctl_xfs_start_commit - prepare to exchange the contents of two files

SYNOPSIS
#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);

DESCRIPTION
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.

RETURN VALUE
On error, -1 is returned, and errno is set to indicate the error.

ERRORS
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.

CONFORMING TO
This API is XFS-specific.

USE CASES

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");

NOTES

Some filesystems may limit the amount of data or the number of extents that can be exchanged in a single call.

SEE ALSO
ioctl(2)