Allocate Disk Space Fast With fallocate

The fallocate command allows you to instantly allocate disk space without having to write to any of the data blocks. It’s a near-instant operation and much easier on your solid-state drive.

When allocating space for a disk images, you might be used to using something like dd if=/dev/zero of=disk.img bs=2048 count=3906250, but this is really slow and creates unnecessary wear.

If you’re using XFS, ext4, Btrfs, or tmpfs, your filesystem supports the fallocate(2) Linux system call which can allocate space without having to write to the blocks. The space is simple marked as uninitialized and is instantly ready to use.

The fallocate(1) utility is included in the util-linux package in Debian-based distros and acts a command-line front-end to the system call. Instead of the above usage of dd, you would run something like the following:

fallocate -l 8GB disk.img

There is also a truncate(1) utility, a part of the coreutils package that can be used to shrink or extend the size of a file.