#Bash 的 fallocate 命令
fallocate [OPTIONS] FILE
功能
为文件预分配磁盘空间，或取消预分配的空间。
类型
可执行文件（/usr/bin/fallocate），属于 util-linux。
参数
OPTIONS选项：-c,--collapse-range- 折叠（删除）指定区间，并将其后的数据向前移动-d,--dig-holes- 检测和释放空洞，但保留文件大小-i,--insert-range- 在指定偏移处插入一段空洞，并将后续数据向后移动-l,--length LENGTH- 在LENGTH长度内进行操作-n,--keep-size- 保持文件逻辑大小不变-o,--offset OFFSET- 从OFFSET位置之后开始操作-p,--punch-hole- 将指定区域作为空洞释放，此后读取该返回将得到零-v,--verbose- 显示详细的操作信息-x,--posix- 使用 POSIX 兼容的空间预分配方式-z,--zero-range- 将指定区间置零--help- 显示帮助--version- 显示版本
FILE- 要操作的问题
空洞指文件占据但不实际使用的空间，例如一个文件里面有很多连续的零，则可以将该空间释放以作他用。
#示例
$ fallocate -l 10M 1.txt # 为文件分配 10M 空间
$ ls -lh 1.txt # 查看文件大小
-rw-r--r-- 1 primers primers 10M Sep 21 10:42 1.txt
$ du -h 1.txt # 查看文件实际占用的大小
10M 1.txt
$ fallocate -d 1.txt # 释放空洞
$ ls -lh 1.txt # 查看文件大小
-rw-r--r-- 1 primers primers 10M Sep 21 10:42 1.txt
$ du -h 1.txt # 查看文件实际占用的大小
4.0K 1.txt
#推荐阅读
#手册
FALLOCATE(1) User Commands FALLOCATE(1)
NAME
fallocate - preallocate or deallocate space to a file
SYNOPSIS
fallocate [-c|-p|-z] [-o offset] -l length [-n] filename
fallocate -d [-o offset] [-l length] filename
fallocate -x [-o offset] -l length filename
DESCRIPTION
fallocate is used to manipulate the allocated disk space for a file,
either to deallocate or preallocate it. For filesystems which support
the fallocate(2) system call, preallocation is done quickly by
allocating blocks and marking them as uninitialized, requiring no IO to
the data blocks. This is much faster than creating a file by filling it
with zeroes.
The exit status returned by fallocate is 0 on success and 1 on failure.
OPTIONS
The length and offset arguments may be followed by the multiplicative
suffixes KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB,
EiB, ZiB, and YiB (the "iB" is optional, e.g., "K" has the same meaning
as "KiB") or the suffixes KB (=1000), MB (=1000*1000), and so on for
GB, TB, PB, EB, ZB, and YB.
The options --collapse-range, --dig-holes, --punch-hole, and
--zero-range are mutually exclusive.
-c, --collapse-range
Removes a byte range from a file, without leaving a hole. The byte
range to be collapsed starts at offset and continues for length
bytes. At the completion of the operation, the contents of the file
starting at the location offset+length will be appended at the
location offset, and the file will be length bytes smaller. The
option --keep-size may not be specified for the collapse-range
operation.
Available since Linux 3.15 for ext4 (only for extent-based files)
and XFS.
A filesystem may place limitations on the granularity of the
operation, in order to ensure efficient implementation. Typically,
offset and length must be a multiple of the filesystem logical
block size, which varies according to the filesystem type and
configuration. If a filesystem has such a requirement, the
operation will fail with the error EINVAL if this requirement is
violated.
-d, --dig-holes
Detect and dig holes. This makes the file sparse in-place, without
using extra disk space. The minimum size of the hole depends on
filesystem I/O block size (usually 4096 bytes). Also, when using
this option, --keep-size is implied. If no range is specified by
--offset and --length, then the entire file is analyzed for holes.
You can think of this option as doing a "cp --sparse" and then
renaming the destination file to the original, without the need for
extra disk space.
See --punch-hole for a list of supported filesystems.
-i, --insert-range
Insert a hole of length bytes from offset, shifting existing data.
-l, --length length
Specifies the length of the range, in bytes.
-n, --keep-size
Do not modify the apparent length of the file. This may effectively
allocate blocks past EOF, which can be removed with a truncate.
-o, --offset offset
Specifies the beginning offset of the range, in bytes.
-p, --punch-hole
Deallocates space (i.e., creates a hole) in the byte range starting
at offset and continuing for length bytes. Within the specified
range, partial filesystem blocks are zeroed, and whole filesystem
blocks are removed from the file. After a successful call,
subsequent reads from this range will return zeroes. This option
may not be specified at the same time as the --zero-range option.
Also, when using this option, --keep-size is implied.
Supported for XFS (since Linux 2.6.38), ext4 (since Linux 3.0),
Btrfs (since Linux 3.7), tmpfs (since Linux 3.5) and gfs2 (since
Linux 4.16).
-v, --verbose
Enable verbose mode.
-x, --posix
Enable POSIX operation mode. In that mode allocation operation
always completes, but it may take longer time when fast allocation
is not supported by the underlying filesystem.
-z, --zero-range
Zeroes space in the byte range starting at offset and continuing
for length bytes. Within the specified range, blocks are
preallocated for the regions that span the holes in the file. After
a successful call, subsequent reads from this range will return
zeroes.
Zeroing is done within the filesystem preferably by converting the
range into unwritten extents. This approach means that the
specified range will not be physically zeroed out on the device
(except for partial blocks at the either end of the range), and I/O
is (otherwise) required only to update metadata.
Option --keep-size can be specified to prevent file length
modification.
Available since Linux 3.14 for ext4 (only for extent-based files)
and XFS.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
AUTHORS
Eric Sandeen <[email protected]>, Karel Zak <[email protected]>
SEE ALSO
truncate(1), fallocate(2), posix_fallocate(3)
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The fallocate command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux 2.39.3 2023-10-23 FALLOCATE(1)