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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
IOCTL-FIDEDUPERANGE(2) Linux Programmer's Manual IOCTL-FIDEDUPERANGE(2)
ioctl_fideduperange - share some the data of one file with another
file
#include <sys/ioctl.h>
#include <linux/fs.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 16MiB. 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.
Error codes can be one of, but are not limited to, the following:
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.
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.
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.
This ioctl operation first appeared in Linux 4.5. It was previously
known as BTRFS_IOC_FILE_EXTENT_SAME and was private to Btrfs.
This API is Linux-specific.
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.
ioctl(2)
This page is part of release 4.12 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/.
Linux 2016-12-12 IOCTL-FIDEDUPERANGE(2)
Pages that refer to this page: ioctl(2)