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.TH IOCTL-FIDEDUPERANGE 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

ioctl_fideduperange \- share some the data of one file with another file

.SH SYNOPSIS

.br

.B #include <sys/ioctl.h>

.br

.B #include <linux/fs.h>

.PP

.BI "int ioctl(int " src_fd ", FIDEDUPERANGE, struct file_dedupe_range *" arg );

.SH DESCRIPTION

If a filesystem supports files sharing physical storage between multiple

files, this

.BR ioctl (2)

operation can be used to make some of the data in the

.B src_fd

file appear in the

.B 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".

.PP

This ioctl performs the "compare and share if identical" operation on up to

.IR src_length

bytes from file descriptor

.IR src_fd

at offset

.IR src_offset ".

This information is conveyed in a structure of the following form:

.PP

.in +4n

.EX

struct file_dedupe_range {

    __u64 src_offset;

    __u64 src_length;

    __u16 dest_count;

    __u16 reserved1;

    __u32 reserved2;

    struct file_dedupe_range_info info[0];

};

.EE

.in

.PP

Deduplication is atomic with regards to concurrent writes, so no locks need to

be taken to obtain a consistent deduplicated copy.

.PP

The fields

.IR reserved1 " and " reserved2

must be zero.

.PP

Destinations for the deduplication operation are conveyed in the array at the

end of the structure.

The number of destinations is given in

.IR dest_count ",

and the destination information is conveyed in the following form:

.PP

.in +4n

.EX

struct file_dedupe_range_info {

    __s64 dest_fd;

    __u64 dest_offset;

    __u64 bytes_deduped;

    __s32 status;

    __u32 reserved;

};

.EE

.in

.PP

Each deduplication operation targets

.IR src_length

bytes in file descriptor

.IR dest_fd

at offset

.IR dest_offset ".

The field

.IR reserved

must be zero.

During the call,

.IR src_fd

must be open for reading and

.IR dest_fd

must be open for writing.

The combined size of the struct

.IR file_dedupe_range

and the struct

.IR file_dedupe_range_info

array must not exceed the system page size.

The maximum size of

.IR 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

.IR src_fd

is mapped into

.IR dest_fd

and the previous contents in

.IR dest_fd

are freed.

.PP

Upon successful completion of this ioctl, the number of bytes successfully

deduplicated is returned in

.IR bytes_deduped

and a status code for the deduplication operation is returned in

.IR status ".

If even a single byte in the range does not match, the deduplication

request will be ignored and

.IR status

set to

.BR FILE_DEDUPE_RANGE_DIFFERS .

The

.IR status

code is set to

.B FILE_DEDUPE_RANGE_SAME

for success, a negative error code in case of error, or

.B FILE_DEDUPE_RANGE_DIFFERS

if the data did not match.

.PP

.SH RETURN VALUE

On error, \-1 is returned, and

.I errno

is set to indicate the error.

.PP

.SH ERRORS

Error codes can be one of, but are not limited to, the following:

.TP

.B ENOMEM

The kernel was unable to allocate sufficient memory to perform the

operation or

.IR dest_count

is so large that the input argument description spans more than a single

page of memory.

.TP

.B EBADF

.IR src_fd

is not open for reading;

.IR dest_fd

is not open for writing or is open for append-only writes; or the filesystem

which

.IR src_fd

resides on does not support deduplication.

.TP

.B 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.

.TP

.B EISDIR

One of the files is a directory and the filesystem does not support shared

regions in directories.

.TP

.B EOPNOTSUPP

This can appear if the filesystem does not support deduplicating either file

descriptor, or if either file descriptor refers to special inodes.

.TP

.B EPERM

.IR dest_fd

is immutable.

.TP

.B ETXTBSY

One of the files is a swap file.

Swap files cannot share storage.

.TP

.B EXDEV

.IR dest_fd " and " src_fd

are not on the same mounted filesystem.

.SH VERSIONS

This ioctl operation first appeared in Linux 4.5.

It was previously known as

.B BTRFS_IOC_FILE_EXTENT_SAME

and was private to Btrfs.

.SH CONFORMING TO

This API is Linux-specific.

.SH NOTES

Because a copy-on-write operation requires the allocation of new storage, the

.BR fallocate (2)

operation may unshare shared blocks to guarantee that subsequent writes will

not fail because of lack of disk space.

.PP

Some filesystems may limit the amount of data that can be deduplicated in a

single call.

.SH SEE ALSO

.BR ioctl (2)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.