man2/ioctl_fideduperange.2

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  23 .TH IOCTL_FIDEDUPERANGE 2 2019-10-10 "Linux" "Linux Programmer's Manual"
  24 .SH NAME
  25 ioctl_fideduperange \- share some the data of one file with another file
  26 .SH SYNOPSIS
  27 .br
  28 .B #include <sys/ioctl.h>
  29 .br
  30 .B #include <linux/fs.h>
  31 .PP
  32 .BI "int ioctl(int " src_fd ", FIDEDUPERANGE, struct file_dedupe_range *" arg );
  33 .SH DESCRIPTION
  34 If a filesystem supports files sharing physical storage between multiple
  35 files, this
  36 .BR ioctl (2)
  37 operation can be used to make some of the data in the
  38 .B src_fd
  39 file appear in the
  40 .B dest_fd
  41 file by sharing the underlying storage if the file data is identical
  42 ("deduplication").
  43 Both files must reside within the same filesystem.
  44 This reduces storage consumption by allowing the filesystem
  45 to store one shared copy of the data.
  46 If a file write should occur to a shared
  47 region, the filesystem must ensure that the changes remain private to the file
  48 being written.
  49 This behavior is commonly referred to as "copy on write".
  50 .PP
  51 This ioctl performs the "compare and share if identical" operation on up to
  52 .IR src_length
  53 bytes from file descriptor
  54 .IR src_fd
  55 at offset
  56 .IR src_offset ".
  57 This information is conveyed in a structure of the following form:
  58 .PP
  59 .in +4n
  60 .EX
  61 struct file_dedupe_range {
  62     __u64 src_offset;
  63     __u64 src_length;
  64     __u16 dest_count;
  65     __u16 reserved1;
  66     __u32 reserved2;
  67     struct file_dedupe_range_info info[0];
  68 };
  69 .EE
  70 .in
  71 .PP
  72 Deduplication is atomic with regards to concurrent writes, so no locks need to
  73 be taken to obtain a consistent deduplicated copy.
  74 .PP
  75 The fields
  76 .IR reserved1 " and " reserved2
  77 must be zero.
  78 .PP
  79 Destinations for the deduplication operation are conveyed in the array at the
  80 end of the structure.
  81 The number of destinations is given in
  82 .IR dest_count ",
  83 and the destination information is conveyed in the following form:
  84 .PP
  85 .in +4n
  86 .EX
  87 struct file_dedupe_range_info {
  88     __s64 dest_fd;
  89     __u64 dest_offset;
  90     __u64 bytes_deduped;
  91     __s32 status;
  92     __u32 reserved;
  93 };
  94 .EE
  95 .in
  96 .PP
  97 Each deduplication operation targets
  98 .IR src_length
  99 bytes in file descriptor
 100 .IR dest_fd
 101 at offset
 102 .IR dest_offset ".
 103 The field
 104 .IR reserved
 105 must be zero.
 106 During the call,
 107 .IR src_fd
 108 must be open for reading and
 109 .IR dest_fd
 110 must be open for writing.
 111 The combined size of the struct
 112 .IR file_dedupe_range
 113 and the struct
 114 .IR file_dedupe_range_info
 115 array must not exceed the system page size.
 116 The maximum size of
 117 .IR src_length
 118 is filesystem dependent and is typically 16\ MiB.
 119 This limit will be enforced silently by the filesystem.
 120 By convention, the storage used by
 121 .IR src_fd
 122 is mapped into
 123 .IR dest_fd
 124 and the previous contents in
 125 .IR dest_fd
 126 are freed.
 127 .PP
 128 Upon successful completion of this ioctl, the number of bytes successfully
 129 deduplicated is returned in
 130 .IR bytes_deduped
 131 and a status code for the deduplication operation is returned in
 132 .IR status ".
 133 If even a single byte in the range does not match, the deduplication
 134 request will be ignored and
 135 .IR status
 136 set to
 137 .BR FILE_DEDUPE_RANGE_DIFFERS .
 138 The
 139 .IR status
 140 code is set to
 141 .B FILE_DEDUPE_RANGE_SAME
 142 for success, a negative error code in case of error, or
 143 .B FILE_DEDUPE_RANGE_DIFFERS
 144 if the data did not match.
 145 .PP
 146 .SH RETURN VALUE
 147 On error, \-1 is returned, and
 148 .I errno
 149 is set to indicate the error.
 150 .PP
 151 .SH ERRORS
 152 Error codes can be one of, but are not limited to, the following:
 153 .TP
 154 .B EBADF
 155 .IR src_fd
 156 is not open for reading;
 157 .IR dest_fd
 158 is not open for writing or is open for append-only writes; or the filesystem
 159 which
 160 .IR src_fd
 161 resides on does not support deduplication.
 162 .TP
 163 .B EINVAL
 164 The filesystem does not support deduplicating the ranges of the given files.
 165 This error can also appear if either file descriptor represents
 166 a device, FIFO, or socket.
 167 Disk filesystems generally require the offset and length arguments
 168 to be aligned to the fundamental block size.
 169 Neither Btrfs nor XFS support
 170 overlapping deduplication ranges in the same file.
 171 .TP
 172 .B EISDIR
 173 One of the files is a directory and the filesystem does not support shared
 174 regions in directories.
 175 .TP
 176 .B ENOMEM
 177 The kernel was unable to allocate sufficient memory to perform the
 178 operation or
 179 .IR dest_count
 180 is so large that the input argument description spans more than a single
 181 page of memory.
 182 .TP
 183 .B EOPNOTSUPP
 184 This can appear if the filesystem does not support deduplicating either file
 185 descriptor, or if either file descriptor refers to special inodes.
 186 .TP
 187 .B EPERM
 188 .IR dest_fd
 189 is immutable.
 190 .TP
 191 .B ETXTBSY
 192 One of the files is a swap file.
 193 Swap files cannot share storage.
 194 .TP
 195 .B EXDEV
 196 .IR dest_fd " and " src_fd
 197 are not on the same mounted filesystem.
 198 .SH VERSIONS
 199 This ioctl operation first appeared in Linux 4.5.
 200 It was previously known as
 201 .B BTRFS_IOC_FILE_EXTENT_SAME
 202 and was private to Btrfs.
 203 .SH CONFORMING TO
 204 This API is Linux-specific.
 205 .SH NOTES
 206 Because a copy-on-write operation requires the allocation of new storage, the
 207 .BR fallocate (2)
 208 operation may unshare shared blocks to guarantee that subsequent writes will
 209 not fail because of lack of disk space.
 210 .PP
 211 Some filesystems may limit the amount of data that can be deduplicated in a
 212 single call.
 213 .SH SEE ALSO
 214 .BR ioctl (2)