1IOCTL-FIDEDUPERANGE(2)     Linux Programmer's Manual    IOCTL-FIDEDUPERANGE(2)
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NAME

6       ioctl_fideduperange - share some the data of one file with another file
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SYNOPSIS

9       #include <sys/ioctl.h>
10       #include <linux/fs.h>
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12       int ioctl(int src_fd, FIDEDUPERANGE, struct file_dedupe_range *arg);
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DESCRIPTION

15       If  a filesystem supports files sharing physical storage between multi‐
16       ple files, this ioctl(2) operation can be used to make some of the data
17       in the src_fd file appear in the dest_fd file by sharing the underlying
18       storage if the file data is identical  ("deduplication").   Both  files
19       must  reside within the same filesystem.  This reduces storage consump‐
20       tion by allowing the filesystem to store one shared copy of  the  data.
21       If  a  file  write should occur to a shared region, the filesystem must
22       ensure that the changes remain private to the file being written.  This
23       behavior is commonly referred to as "copy on write".
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25       This  ioctl  performs the "compare and share if identical" operation on
26       up to src_length bytes from file descriptor src_fd at  offset  src_off‐
27       set.   This  information  is  conveyed  in a structure of the following
28       form:
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30           struct file_dedupe_range {
31               __u64 src_offset;
32               __u64 src_length;
33               __u16 dest_count;
34               __u16 reserved1;
35               __u32 reserved2;
36               struct file_dedupe_range_info info[0];
37           };
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39       Deduplication is atomic with regards to concurrent writes, so no  locks
40       need to be taken to obtain a consistent deduplicated copy.
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42       The fields reserved1 and reserved2 must be zero.
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44       Destinations  for the deduplication operation are conveyed in the array
45       at the end of the structure.  The number of destinations  is  given  in
46       dest_count,  and the destination information is conveyed in the follow‐
47       ing form:
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49           struct file_dedupe_range_info {
50               __s64 dest_fd;
51               __u64 dest_offset;
52               __u64 bytes_deduped;
53               __s32 status;
54               __u32 reserved;
55           };
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57       Each deduplication operation targets src_length bytes in file  descrip‐
58       tor  dest_fd  at  offset dest_offset.  The field reserved must be zero.
59       During the call, src_fd must be open for reading and  dest_fd  must  be
60       open  for  writing.   The combined size of the struct file_dedupe_range
61       and the struct file_dedupe_range_info array must not exceed the  system
62       page  size.  The maximum size of src_length is filesystem dependent and
63       is typically 16 MiB.  This limit  will  be  enforced  silently  by  the
64       filesystem.   By  convention, the storage used by src_fd is mapped into
65       dest_fd and the previous contents in dest_fd are freed.
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67       Upon successful completion of this ioctl, the number of bytes  success‐
68       fully  deduplicated  is returned in bytes_deduped and a status code for
69       the deduplication operation is returned in status.  If  even  a  single
70       byte  in  the  range  does not match, the deduplication request will be
71       ignored and status set to FILE_DEDUPE_RANGE_DIFFERS.  The  status  code
72       is  set to FILE_DEDUPE_RANGE_SAME for success, a negative error code in
73       case of error, or FILE_DEDUPE_RANGE_DIFFERS if the data did not match.
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RETURN VALUE

76       On error, -1 is returned, and errno is set to indicate the error.
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ERRORS

79       Error codes can be one of, but are not limited to, the following:
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81       ENOMEM The kernel was unable to allocate sufficient memory  to  perform
82              the  operation or dest_count is so large that the input argument
83              description spans more than a single page of memory.
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85       EBADF  src_fd is not open for reading; dest_fd is not open for  writing
86              or  is  open  for  append-only  writes;  or the filesystem which
87              src_fd resides on does not support deduplication.
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89       EINVAL The filesystem does not support deduplicating the ranges of  the
90              given files.  This error can also appear if either file descrip‐
91              tor represents a device, FIFO, or socket.  Disk filesystems gen‐
92              erally  require the offset and length arguments to be aligned to
93              the fundamental block size.  Neither Btrfs nor XFS support over‐
94              lapping deduplication ranges in the same file.
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96       EISDIR One of the files is a directory and the filesystem does not sup‐
97              port shared regions in directories.
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99       EOPNOTSUPP
100              This can appear if the filesystem does not support deduplicating
101              either  file  descriptor, or if either file descriptor refers to
102              special inodes.
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104       EPERM  dest_fd is immutable.
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106       ETXTBSY
107              One of the files is a swap file.  Swap files cannot share  stor‐
108              age.
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110       EXDEV  dest_fd and src_fd are not on the same mounted filesystem.
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VERSIONS

113       This  ioctl  operation  first appeared in Linux 4.5.  It was previously
114       known as BTRFS_IOC_FILE_EXTENT_SAME and was private to Btrfs.
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CONFORMING TO

117       This API is Linux-specific.
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NOTES

120       Because a copy-on-write operation requires the allocation of new  stor‐
121       age,  the fallocate(2) operation may unshare shared blocks to guarantee
122       that subsequent writes will not fail because of lack of disk space.
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124       Some filesystems may limit the amount of data that can be  deduplicated
125       in a single call.
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SEE ALSO

128       ioctl(2)
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COLOPHON

131       This  page  is  part of release 4.15 of the Linux man-pages project.  A
132       description of the project, information about reporting bugs,  and  the
133       latest     version     of     this    page,    can    be    found    at
134       https://www.kernel.org/doc/man-pages/.
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138Linux                             2017-09-15            IOCTL-FIDEDUPERANGE(2)
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