sys-utils/fallocate.1

   1 .TH FALLOCATE 1 "September 2011" "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 ]
   7 .RB [ \-n ]
   8 .RB [ \-p ]
   9 .RB [ \-z ]
  10 .RB [ \-o
  11 .IR offset ]
  12 .B \-l
  13 .IR length
  14 .I filename
  15 .PP
  16 .B fallocate \-d
  17 .RB [ \-o
  18 .IR offset ]
  19 .RB [ \-l
  20 .IR length ]
  21 .I filename
  22 .SH DESCRIPTION
  23 .B fallocate
  24 is used to manipulate the allocated disk space for a file, either to deallocate
  25 or preallocate it.  For filesystems which support the fallocate system call,
  26 preallocation is done quickly by allocating blocks and marking them as
  27 uninitialized, requiring no IO to the data blocks.  This is much faster than
  28 creating a file by filling it with zeros.
  29 .PP
  30 The exit code returned by
  31 .B fallocate
  32 is 0 on success and 1 on failure.
  33 .SH OPTIONS
  34 The \fIlength\fR and \fIoffset\fR
  35 arguments may be followed by the multiplicative suffixes KiB (=1024),
  36 MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is
  37 optional, e.g., "K" has the same meaning as "KiB") or the suffixes
  38 KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.
  39 .PP
  40 The options \fB\-\-collapse\-range\fP, \fB\-\-dig\-holes\fP, \fB\-\-punch\-hole\fP and
  41 \fB\-\-zero\-range\fP are mutually exclusive.
  42 .TP
  43 .BR \-c , " \-\-collapse\-range"
  44 Removes a byte range from a file, without leaving a hole.  The byte range
  45 to be collapsed starts at \fIoffset\fP and continues
  46 for \fIlength\fR bytes.  At the completion of the operation, the contents of
  47 the file starting at the location \fIoffset\fR+\fIlength\fR will be appended at the
  48 location \fIoffset\fR, and the file will be \fIlength\fR bytes smaller.  The option
  49 \fB\-\-keep\-size\fR may not be specified for colapse range operation.
  50 .sp
  51 Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.
  52 .TP
  53 .BR \-d , " \-\-dig\-holes"
  54 Detect and dig holes.  This makes the file sparse in-place, without using extra
  55 disk space.  The minimum size of the hole depends on filesystem I/O block size
  56 (usually 4096 bytes).  Also, when using this option, \fB\-\-keep\-size\fP is
  57 implied.  If no range is specified by \fB\-\-offset\fP and \fB\-\-length\fP,
  58 then the entire file is analyzed for holes.
  59 .sp
  60 You can think of this option as doing a "\fBcp --sparse\fP" and then renaming
  61 the destination file to the original, without the need for extra disk space.
  62 .sp
  63 See \fB\-\-punch\-hole\fP for a list of supported filesystems.
  64 .TP
  65 .BR \-l , " \-\-length " \fIlength
  66 Specifies the length of the range, in bytes.
  67 .TP
  68 .BR \-n , " \-\-keep\-size"
  69 Do not modify the apparent length of the file.  This may effectively allocate
  70 blocks past EOF, which can be removed with a truncate.
  71 .TP
  72 .BR \-o , " \-\-offset " \fIoffset
  73 Specifies the beginning offset of the range, in bytes.
  74 .TP
  75 .BR \-p , " \-\-punch\-hole"
  76 Deallocates space (i.e., creates a hole) in the byte range starting at
  77 \fIoffset\fP and continuing for \fIlength\fR bytes.  Within the
  78 specified range, partial filesystem blocks are zeroed, and whole
  79 filesystem blocks are removed from the file.  After a successful
  80 call, subsequent reads from this range will return zeroes.  This option
  81 may not be specified at the same time as  the \fB\-\-zero\-range\fP option.
  82 Also, when using this option, \fB\-\-keep\-size\fP is implied.
  83 .sp
  84 Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0),
  85 Btrfs (since Linux 3.7) and tmpfs (since Linux 3.5).
  86 .TP
  87 .BR \-v , " \-\-verbose"
  88 Enable verbose mode.
  89 .TP
  90 .BR \-z , " \-\-zero\-range"
  91 Zeroes space in the byte range starting at \fIoffset\fP and
  92 continuing for \fIlength\fR bytes.  Within the specified range, blocks are
  93 preallocated for the regions that span the holes in the file.  After
  94 a successful call, subsequent reads from this range will return zeroes.
  95 .sp
  96 Zeroing is done within the filesystem preferably by converting the
  97 range into unwritten extents.  This approach means that the specified
  98 range will not be physically zeroed out on the device (except for
  99 partial blocks at the either end of the range), and I/O is
 100 (otherwise) required only to update metadata.
 101 .sp
 102 Option \fB\-\-keep\-size\fP can be specified to prevent file length
 103 modification.
 104 .sp
 105 Available since Linux 3.14 for ext4 (only for extent-based files) and XFS.
 106 .TP
 107 .BR \-V , " \-\-version"
 108 Display version information and exit.
 109 .TP
 110 .BR \-h , " \-\-help"
 111 Display help text and exit.
 112 .SH AUTHORS
 113 .UR sandeen@redhat.com
 114 Eric Sandeen
 115 .UE
 116 .br
 117 .UR kzak@redhat.com
 118 Karel Zak
 119 .UE
 120 .SH SEE ALSO
 121 .BR fallocate (2),
 122 .BR posix_fallocate (3),
 123 .BR truncate (1)
 124 .SH AVAILABILITY
 125 The fallocate command is part of the util-linux package and is available from
 126 .UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 127 Linux Kernel Archive
 128 .UE .