Using fallocate instead of dd for creating empty files

I've been always using dd for empty disk image creation (for new virtual machine for example). But I've found out a better tool. It called fallocate.

Usually when I want to create disk image I'll use something like this:

[root@kra images]$ dd if=/dev/zero of=disk.img bs=1M count=2048
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 3.82779 s, 561 MB/s

When I need bigger image it takes a lot of time. And my computer isn't happy during image creation because of higher IO load. I can deal with I/O load using ionice:

[root@kra images]$ ionice -c 3 dd if=/dev/zero of=disk.img bs=1M count=2048

This will put dd to idle scheduler class. Processes in idle class gets to IO operation only when no other process in other classes needs IO. Further I can limit IO rate with throttle:

[root@kra images]# dd if=/dev/zero bs=1M count=2048 | throttle -M 300 > disk.img 
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 7.807 s, 275 MB/s

But best solution is to use fallocate utility which comes from util-linux package. It will create the file with instant speed:

[root@kra images]# rm disk.img 
[root@kra images]# time fallocate -l 2048MiB disk.img

real    0m0.001s
user    0m0.000s
sys     0m0.001s
[root@kra images]# du -sm disk.img 
2048    disk.img

One disadvantage is that this feature may not be supported on all filesystems. According to man page following filesystems are supported: btrfs, ext4, ocfs2, and xfs.

Sources

• man fallocate
• man ionice
• man throttle