mirror of https://github.com/mkerrisk/man-pages
216 lines
5.9 KiB
Groff
216 lines
5.9 KiB
Groff
.\" Copyright (c) 2016, Oracle. All rights reserved.
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.TH IOCTL-FIDEDUPERANGE 2 2016-12-12 "Linux" "Linux Programmer's Manual"
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.SH NAME
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ioctl_fideduperange \- share some the data of one file with another file
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.SH SYNOPSIS
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.br
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.B #include <sys/ioctl.h>
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.br
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.B #include <linux/fs.h>
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.sp
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.BI "int ioctl(int " src_fd ", FIDEDUPERANGE, struct file_dedupe_range *" arg );
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.SH DESCRIPTION
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If a filesystem supports files sharing physical storage between multiple
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files, this
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.BR ioctl (2)
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operation can be used to make some of the data in the
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.B src_fd
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file appear in the
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.B dest_fd
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file by sharing the underlying storage if the file data is identical
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("deduplication").
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Both files must reside within the same filesystem.
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This reduces storage consumption by allowing the filesystem
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to store one shared copy of the data.
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If a file write should occur to a shared
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region, the filesystem must ensure that the changes remain private to the file
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being written.
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This behavior is commonly referred to as "copy on write".
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This ioctl performs the "compare and share if identical" operation on up to
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.IR src_length
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bytes from file descriptor
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.IR src_fd
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at offset
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.IR src_offset ".
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This information is conveyed in a structure of the following form:
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.in +4n
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.nf
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struct file_dedupe_range {
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__u64 src_offset;
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__u64 src_length;
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__u16 dest_count;
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__u16 reserved1;
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__u32 reserved2;
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struct file_dedupe_range_info info[0];
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};
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.fi
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.in
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Deduplication is atomic with regards to concurrent writes, so no locks need to
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be taken to obtain a consistent deduplicated copy.
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The fields
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.IR reserved1 " and " reserved2
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must be zero.
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Destinations for the deduplication operation are conveyed in the array at the
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end of the structure.
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The number of destinations is given in
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.IR dest_count ",
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and the destination information is conveyed in the following form:
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.in +4n
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.nf
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struct file_dedupe_range_info {
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__s64 dest_fd;
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__u64 dest_offset;
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__u64 bytes_deduped;
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__s32 status;
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__u32 reserved;
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};
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.fi
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.in
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Each deduplication operation targets
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.IR src_length
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bytes in file descriptor
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.IR dest_fd
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at offset
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.IR dest_offset ".
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The field
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.IR reserved
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must be zero.
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During the call,
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.IR src_fd
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must be open for reading and
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.IR dest_fd
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must be open for writing.
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The combined size of the struct
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.IR file_dedupe_range
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and the struct
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.IR file_dedupe_range_info
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array must not exceed the system page size.
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The maximum size of
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.IR src_length
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is filesystem dependent and is typically 16MiB.
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This limit will be enforced silently by the filesystem.
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By convention, the storage used by
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.IR src_fd
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is mapped into
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.IR dest_fd
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and the previous contents in
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.IR dest_fd
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are freed.
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Upon successful completion of this ioctl, the number of bytes successfully
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deduplicated is returned in
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.IR bytes_deduped
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and a status code for the deduplication operation is returned in
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.IR status ".
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If even a single byte in the range does not match, the deduplication
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request will be ignored and
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.IR status
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set to
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.BR FILE_DEDUPE_RANGE_DIFFERS .
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The
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.IR status
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code is set to
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.B FILE_DEDUPE_RANGE_SAME
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for success, a negative error code in case of error, or
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.B FILE_DEDUPE_RANGE_DIFFERS
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if the data did not match.
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.SH RETURN VALUE
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.PP
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.SH ERRORS
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Error codes can be one of, but are not limited to, the following:
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.TP
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.B ENOMEM
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The kernel was unable to allocate sufficient memory to perform the
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operation or
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.IR dest_count
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is so large that the input argument description spans more than a single
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page of memory.
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.TP
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.B EBADF
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.IR src_fd
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is not open for reading;
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.IR dest_fd
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is not open for writing or is open for append-only writes; or the filesystem
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which
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.IR src_fd
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resides on does not support deduplication.
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.TP
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.B EINVAL
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The filesystem does not support deduplicating the ranges of the given files.
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This error can also appear if either file descriptor represents
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a device, FIFO, or socket.
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Disk filesystems generally require the offset and length arguments
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to be aligned to the fundamental block size.
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Neither Btrfs nor XFS support
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overlapping deduplication ranges in the same file.
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.TP
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.B EISDIR
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One of the files is a directory and the filesystem does not support shared
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regions in directories.
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.TP
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.B EOPNOTSUPP
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This can appear if the filesystem does not support deduplicating either file
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descriptor, or if either file descriptor refers to special inodes.
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.TP
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.B EPERM
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.IR dest_fd
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is immutable.
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.TP
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.B ETXTBSY
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One of the files is a swap file.
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Swap files cannot share storage.
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.TP
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.B EXDEV
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.IR dest_fd " and " src_fd
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are not on the same mounted filesystem.
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.SH VERSIONS
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This ioctl operation first appeared in Linux 4.5.
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It was previously known as
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.B BTRFS_IOC_FILE_EXTENT_SAME
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and was private to Btrfs.
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.SH CONFORMING TO
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This API is Linux-specific.
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.SH NOTES
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Because a copy-on-write operation requires the allocation of new storage, the
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.BR fallocate (2)
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operation may unshare shared blocks to guarantee that subsequent writes will
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not fail because of lack of disk space.
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Some filesystems may limit the amount of data that can be deduplicated in a
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single call.
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.SH SEE ALSO
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.BR ioctl (2)
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