fallocate — preallocate space to a file


fallocate [−n] [−p] [ −o offset ] −l length filename


fallocate is used to preallocate blocks to a file. For filesystems which support the fallocate system call, this 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.

As of the Linux Kernel v2.6.31, the fallocate system call is supported by the btrfs, ext4, ocfs2, and xfs filesystems.

The exit code returned by fallocate is 0 on success and 1 on failure.


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.

−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.

−p, −−punch−hole

Punch holes in the file, the range should not exceed the length of the file.

−o, −−offset offset

Specifies the beginning offset of the allocation, in bytes.

−l, −−length length

Specifies the length of the allocation, in bytes.

−h, −−help

Print help and exit.

−V, −−version

Print version and exit.


Eric Sandeen

Karel Zak


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


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