317 lines
7.4 KiB
HTML
317 lines
7.4 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of IOCTL-FIDEDUPERANGE</TITLE>
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</HEAD><BODY>
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<H1>IOCTL-FIDEDUPERANGE</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-10-10<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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ioctl_fideduperange - share some the data of one file with another file
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<BR>
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<B>#include <<A HREF="file:///usr/include/sys/ioctl.h">sys/ioctl.h</A>></B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/linux/fs.h">linux/fs.h</A>></B>
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<P>
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<B>int ioctl(int </B><I>src_fd</I><B>, FIDEDUPERANGE, struct file_dedupe_range *</B><I>arg</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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If a filesystem supports files sharing physical storage between multiple
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files, this
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl">ioctl</A></B>(2)
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operation can be used to make some of the data in the
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<B>src_fd</B>
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file appear in the
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<B>dest_fd</B>
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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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<P>
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This ioctl performs the "compare and share if identical" operation on up to
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<I>src_length</I>
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bytes from file descriptor
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<I>src_fd</I>
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at offset
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<I>src_offset</I>.
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This information is conveyed in a structure of the following form:
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<P>
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struct file_dedupe_range {
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<BR> __u64 src_offset;
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<BR> __u64 src_length;
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<BR> __u16 dest_count;
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<BR> __u16 reserved1;
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<BR> __u32 reserved2;
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<BR> struct file_dedupe_range_info info[0];
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};
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<P>
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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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<P>
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The fields
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<I>reserved1</I> and <I>reserved2</I>
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must be zero.
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<P>
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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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<I>dest_count</I>,
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and the destination information is conveyed in the following form:
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<P>
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struct file_dedupe_range_info {
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<BR> __s64 dest_fd;
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<BR> __u64 dest_offset;
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<BR> __u64 bytes_deduped;
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<BR> __s32 status;
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<BR> __u32 reserved;
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};
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<P>
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Each deduplication operation targets
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<I>src_length</I>
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bytes in file descriptor
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<I>dest_fd</I>
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at offset
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<I>dest_offset</I>.
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The field
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<I>reserved</I>
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must be zero.
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During the call,
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<I>src_fd</I>
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must be open for reading and
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<I>dest_fd</I>
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must be open for writing.
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The combined size of the struct
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<I>file_dedupe_range</I>
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and the struct
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<I>file_dedupe_range_info</I>
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array must not exceed the system page size.
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The maximum size of
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<I>src_length</I>
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is filesystem dependent and is typically 16 MiB.
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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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<I>src_fd</I>
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is mapped into
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<I>dest_fd</I>
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and the previous contents in
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<I>dest_fd</I>
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are freed.
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<P>
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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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<I>bytes_deduped</I>
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and a status code for the deduplication operation is returned in
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<I>status</I>.
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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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<I>status</I>
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set to
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<B>FILE_DEDUPE_RANGE_DIFFERS</B>.
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The
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<I>status</I>
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code is set to
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<B>FILE_DEDUPE_RANGE_SAME</B>
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for success, a negative error code in case of error, or
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<B>FILE_DEDUPE_RANGE_DIFFERS</B>
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if the data did not match.
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<P>
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On error, -1 is returned, and
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<I>errno</I>
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is set to indicate the error.
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<P>
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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Error codes can be one of, but are not limited to, the following:
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<DL COMPACT>
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<DT id="1"><B>EBADF</B>
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<DD>
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<I>src_fd</I>
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is not open for reading;
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<I>dest_fd</I>
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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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<I>src_fd</I>
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resides on does not support deduplication.
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<DT id="2"><B>EINVAL</B>
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<DD>
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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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<DT id="3"><B>EISDIR</B>
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<DD>
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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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<DT id="4"><B>ENOMEM</B>
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<DD>
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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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<I>dest_count</I>
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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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<DT id="5"><B>EOPNOTSUPP</B>
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<DD>
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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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<DT id="6"><B>EPERM</B>
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<DD>
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<I>dest_fd</I>
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is immutable.
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<DT id="7"><B>ETXTBSY</B>
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<DD>
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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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<DT id="8"><B>EXDEV</B>
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<DD>
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<I>dest_fd</I> and <I>src_fd</I>
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are not on the same mounted filesystem.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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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</B>
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and was private to Btrfs.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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This API is Linux-specific.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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Because a copy-on-write operation requires the allocation of new storage, the
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<B><A HREF="/cgi-bin/man/man2html?2+fallocate">fallocate</A></B>(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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<P>
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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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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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|
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl">ioctl</A></B>(2)
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="9"><A HREF="#lbAB">NAME</A><DD>
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<DT id="10"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="11"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="12"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="13"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="14"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="15"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="16"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="17"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="18"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:33 GMT, March 31, 2021
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</BODY>
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</HTML>
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