FIDEDUPERANGE(2const) - Linux manual page (original) (raw)
FIDEDUPERANGE(2const) FIDEDUPERANGE(2const)
NAME top
FIDEDUPERANGE - share some the data of one file with another fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <linux/fs.h>** /* Definition of **FIDEDUPERANGE** and
**FILE_DEDUPE_*** constants***/**
**#include <sys/ioctl.h>**
**int ioctl(int** _srcfd_**, FIDEDUPERANGE, struct file_dedupe_range ***_arg_**);**DESCRIPTION top
If a filesystem supports files sharing physical storage between
multiple files, this [ioctl(2)](../man2/ioctl.2.html) 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 _srclength_ bytes from file descriptor _srcfd_ at offset
_srcoffset_. 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 _destcount_, 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 _srclength_ bytes in file
descriptor _destfd_ at offset _destoffset_. The field _reserved_ must
be zero. During the call, _srcfd_ must be open for reading and
_destfd_ must be open for writing. The combined size of the struct
_filededuperange_ and the struct _filededuperangeinfo_ array must
not exceed the system page size. The maximum size of _srclength_
is filesystem dependent and is typically 16 MiB. This limit will
be enforced silently by the filesystem. By convention, the
storage used by _srcfd_ is mapped into _destfd_ and the previous
contents in _destfd_ are freed.
Upon successful completion of this ioctl, the number of bytes
successfully deduplicated is returned in _bytesdeduped_ 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 operation 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.RETURN VALUE top
On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
Possible errors include (but are not limited to) the following:
**EBADF** _srcfd_ is not open for reading; _destfd_ is not open for
writing or is open for append-only writes; or the
filesystem which _srcfd_ 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 _destcount_ 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** _destfd_ is immutable.
**ETXTBSY**
One of the files is a swap file. Swap files cannot share
storage.
**EXDEV** _destfd_ and _srcfd_ are not on the same mounted filesystem.VERSIONS top
Some filesystems may limit the amount of data that can be
deduplicated in a single call.STANDARDS top
Linux.HISTORY top
Linux 4.5.
It was previously known as **BTRFS_IOC_FILE_EXTENT_SAME** and was
private to Btrfs.NOTES top
Because a copy-on-write operation requires the allocation of new
storage, the [fallocate(2)](../man2/fallocate.2.html) operation may unshare shared blocks to
guarantee that subsequent writes will not fail because of lack of
disk space.SEE ALSO top
[ioctl(2)](../man2/ioctl.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 FIDEDUPERANGE(2const)
Pages that refer to this page:ioctl_fs(2)