sys-utils/fallocate.1

   1 .TH FALLOCATE 1 "April 2014" "util-linux" "User Commands"
   2 .SH NAME
   3 fallocate \- preallocate or deallocate space to a file
   4 .SH SYNOPSIS
   5 .B fallocate
   6 .RB [ \-c | \-p | \-z ]
   7 .RB [ \-o
   8 .IR offset ]
   9 .B \-l
  10 .IR length
  11 .RB [ \-n ]
  12 .I filename
  13 .PP
  14 .B fallocate \-d
  15 .RB [ \-o
  16 .IR offset ]
  17 .RB [ \-l
  18 .IR length ]
  19 .I filename
  20 .PP
  21 .B fallocate \-x
  22 .RB [ \-o
  23 .IR offset ]
  24 .RB \-l
  25 .IR length
  26 .I filename
  27 .SH DESCRIPTION
  28 .B fallocate
  29 is used to manipulate the allocated disk space for a file, either to deallocate
  30 or preallocate it.  For filesystems which support the fallocate system call,
  31 preallocation is done quickly by allocating blocks and marking them as
  32 uninitialized, requiring no IO to the data blocks.  This is much faster than
  33 creating a file by filling it with zeroes.
  34 .PP
  35 The exit code returned by
  36 .B fallocate
  37 is 0 on success and 1 on failure.
  38 .SH OPTIONS
  39 The \fIlength\fR and \fIoffset\fR
  40 arguments may be followed by the multiplicative suffixes KiB (=1024),
  41 MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is
  42 optional, e.g., "K" has the same meaning as "KiB") or the suffixes
  43 KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.
  44 .PP
  45 The options \fB\-\-collapse\-range\fP, \fB\-\-dig\-holes\fP, \fB\-\-punch\-hole\fP and
  46 \fB\-\-zero\-range\fP are mutually exclusive.
  47 .TP
  48 .BR \-c , " \-\-collapse\-range"
  49 Removes a byte range from a file, without leaving a hole.  The byte range
  50 to be collapsed starts at \fIoffset\fP and continues
  51 for \fIlength\fR bytes.  At the completion of the operation, the contents of
  52 the file starting at the location \fIoffset\fR+\fIlength\fR will be appended at the
  53 location \fIoffset\fR, and the file will be \fIlength\fR bytes smaller.  The option
  54 \fB\-\-keep\-size\fR may not be specified for the collapse-range operation.
  55 .sp
  56 Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.
  57 .TP
  58 .BR \-d , " \-\-dig\-holes"
  59 Detect and dig holes.  This makes the file sparse in-place, without using extra
  60 disk space.  The minimum size of the hole depends on filesystem I/O block size
  61 (usually 4096 bytes).  Also, when using this option, \fB\-\-keep\-size\fP is
  62 implied.  If no range is specified by \fB\-\-offset\fP and \fB\-\-length\fP,
  63 then the entire file is analyzed for holes.
  64 .sp
  65 You can think of this option as doing a "\fBcp --sparse\fP" and then renaming
  66 the destination file to the original, without the need for extra disk space.
  67 .sp
  68 See \fB\-\-punch\-hole\fP for a list of supported filesystems.
  69 .TP
  70 .BR \-i , " \-\-insert\-range"
  71 Insert a hole of \fIlength\fR bytes from \fIoffset\fR, shifting existing data.
  72 .TP
  73 .BR \-l , " \-\-length " \fIlength
  74 Specifies the length of the range, in bytes.
  75 .TP
  76 .BR \-n , " \-\-keep\-size"
  77 Do not modify the apparent length of the file.  This may effectively allocate
  78 blocks past EOF, which can be removed with a truncate.
  79 .TP
  80 .BR \-o , " \-\-offset " \fIoffset
  81 Specifies the beginning offset of the range, in bytes.
  82 .TP
  83 .BR \-p , " \-\-punch\-hole"
  84 Deallocates space (i.e., creates a hole) in the byte range starting at
  85 \fIoffset\fP and continuing for \fIlength\fR bytes.  Within the
  86 specified range, partial filesystem blocks are zeroed, and whole
  87 filesystem blocks are removed from the file.  After a successful
  88 call, subsequent reads from this range will return zeroes.  This option
  89 may not be specified at the same time as  the \fB\-\-zero\-range\fP option.
  90 Also, when using this option, \fB\-\-keep\-size\fP is implied.
  91 .sp
  92 Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0),
  93 Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).
  94 .TP
  95 .BR \-v , " \-\-verbose"
  96 Enable verbose mode.
  97 .TP
  98 .BR \-x , " \-\-posix"
  99 Enable POSIX operation mode. In that mode allocation operation always completes,
 100 but it may take longer time when fast allocation is not supported by the underlying filesystem.
 101 .TP
 102 .BR \-z , " \-\-zero\-range"
 103 Zeroes space in the byte range starting at \fIoffset\fP and
 104 continuing for \fIlength\fR bytes.  Within the specified range, blocks are
 105 preallocated for the regions that span the holes in the file.  After
 106 a successful call, subsequent reads from this range will return zeroes.
 107 .sp
 108 Zeroing is done within the filesystem preferably by converting the
 109 range into unwritten extents.  This approach means that the specified
 110 range will not be physically zeroed out on the device (except for
 111 partial blocks at the either end of the range), and I/O is
 112 (otherwise) required only to update metadata.
 113 .sp
 114 Option \fB\-\-keep\-size\fP can be specified to prevent file length
 115 modification.
 116 .sp
 117 Available since Linux 3.14 for ext4 (only for extent-based files) and XFS.
 118 .TP
 119 .BR \-V , " \-\-version"
 120 Display version information and exit.
 121 .TP
 122 .BR \-h , " \-\-help"
 123 Display help text and exit.
 124 .SH AUTHORS
 125 .UR sandeen@redhat.com
 126 Eric Sandeen
 127 .UE
 128 .br
 129 .UR kzak@redhat.com
 130 Karel Zak
 131 .UE
 132 .SH SEE ALSO
 133 .BR truncate (1),
 134 .BR fallocate (2),
 135 .BR posix_fallocate (3)
 136 .SH AVAILABILITY
 137 The fallocate command is part of the util-linux package and is available from
 138 .UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 139 Linux Kernel Archive
 140 .UE .