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FALLOCATE

Section: User Commands (1)
Updated: September 2011
Index Return to Main Contents
 

NAME

fallocate - preallocate or deallocate space to a file  

SYNOPSIS

fallocate [-c] [-n] [-p] [-z] [-o offset] -l length filename

fallocate -d [-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 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 zeros.

The exit code 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 colapse range operation.

Available since Linux 3.15 for ext4 (only for extent-based files) and XFS.

-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.

-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) and tmpfs (since Linux 3.5).

-v, --verbose
Enable verbose mode.
-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.

-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
 

AUTHORS

Eric Sandeen
Karel Zak  

SEE ALSO

fallocate(2), posix_fallocate(3), truncate(1)  

AVAILABILITY

The fallocate command is part of the util-linux package and is available from Linux Kernel Archive


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
AUTHORS
SEE ALSO
AVAILABILITY


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