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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO |
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ioctl_fideduperange(2) System Calls Manual ioctl_fideduperange(2)
ioctl_fideduperange - share some the data of one file with
another file
Standard C library (libc, -lc)
#include <linux/fs.h> /* Definition of FIDEDUPERANGE and
FILE_DEDUPE_* constants*/
#include <sys/ioctl.h>
int ioctl(int src_fd, FIDEDUPERANGE, struct file_dedupe_range *arg);
If a filesystem supports files sharing physical storage between
multiple files, this ioctl(2) operation can be used to make some
of the data in the src_fd file appear in the dest_fd file by
sharing the underlying storage if the file data is identical
("deduplication"). Both files must reside within the same
filesystem. This reduces storage consumption by allowing the
filesystem to store one shared copy of the data. If a file write
should occur to a shared region, the filesystem must ensure that
the changes remain private to the file being written. This
behavior is commonly referred to as "copy on write".
This ioctl performs the "compare and share if identical"
operation on up to src_length bytes from file descriptor src_fd
at offset src_offset. This information is conveyed in a
structure of the following form:
struct file_dedupe_range {
__u64 src_offset;
__u64 src_length;
__u16 dest_count;
__u16 reserved1;
__u32 reserved2;
struct file_dedupe_range_info info[0];
};
Deduplication is atomic with regards to concurrent writes, so no
locks need to be taken to obtain a consistent deduplicated copy.
The fields reserved1 and reserved2 must be zero.
Destinations for the deduplication operation are conveyed in the
array at the end of the structure. The number of destinations is
given in dest_count, and the destination information is conveyed
in the following form:
struct file_dedupe_range_info {
__s64 dest_fd;
__u64 dest_offset;
__u64 bytes_deduped;
__s32 status;
__u32 reserved;
};
Each deduplication operation targets src_length bytes in file
descriptor dest_fd at offset dest_offset. The field reserved
must be zero. During the call, src_fd must be open for reading
and dest_fd must be open for writing. The combined size of the
struct file_dedupe_range and the struct file_dedupe_range_info
array must not exceed the system page size. The maximum size of
src_length is filesystem dependent and is typically 16 MiB. This
limit will be enforced silently by the filesystem. By
convention, the storage used by src_fd is mapped into dest_fd and
the previous contents in dest_fd are freed.
Upon successful completion of this ioctl, the number of bytes
successfully deduplicated is returned in bytes_deduped and a
status code for the deduplication operation is returned in
status. If even a single byte in the range does not match, the
deduplication request will be ignored and status set to
FILE_DEDUPE_RANGE_DIFFERS. The status code is set to
FILE_DEDUPE_RANGE_SAME for success, a negative error code in case
of error, or FILE_DEDUPE_RANGE_DIFFERS if the data did not match.
On error, -1 is returned, and errno is set to indicate the error.
Possible errors include (but are not limited to) the following:
EBADF src_fd is not open for reading; dest_fd is not open for
writing or is open for append-only writes; or the
filesystem which src_fd resides on does not support
deduplication.
EINVAL The filesystem does not support deduplicating the ranges
of the given 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 size.
Neither Btrfs nor XFS support overlapping deduplication
ranges in the same file.
EISDIR One of the files is a directory and the filesystem does
not support shared regions in directories.
ENOMEM The kernel was unable to allocate sufficient memory to
perform the operation or dest_count is so large that the
input argument description spans more than a single page
of memory.
EOPNOTSUPP
This can appear if the filesystem does not support
deduplicating either file descriptor, or if either file
descriptor refers to special inodes.
EPERM dest_fd is immutable.
ETXTBSY
One of the files is a swap file. Swap files cannot share
storage.
EXDEV dest_fd and src_fd are not on the same mounted filesystem.
Some filesystems may limit the amount of data that can be
deduplicated in a single call.
Linux.
Linux 4.5.
It was previously known as BTRFS_IOC_FILE_EXTENT_SAME and was
private to Btrfs.
Because a copy-on-write operation requires the allocation of new
storage, the fallocate(2) operation may unshare shared blocks to
guarantee that subsequent writes will not fail because of lack of
disk space.
ioctl(2)
Linux man-pages (unreleased) (date) ioctl_fideduperange(2)
Pages that refer to this page: ioctl(2)
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HTML rendering created 2023-12-22 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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