ioctl_fideduperange — share some the data of one file with another file
#include <linux/fs.h> /* Definition of FIDEDUPERANGE and .BR " 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.
Possible errors include (but are not limited to) the following:
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.
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.
One of the files is a directory and the filesystem does not support shared regions in directories.
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.
This can appear if the filesystem does not support deduplicating either file descriptor, or if either file descriptor refers to special inodes.
dest_fd is
immutable.
One of the files is a swap file. Swap files cannot share storage.
dest_fd and
src_fd are not
on the same mounted filesystem.
This ioctl operation first appeared in 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.
Some filesystems may limit the amount of data that can be deduplicated in a single call.
This page is part of release 5.13 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
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