fallocate vs dd for swap file creation

I recently ran across this helpful Digital Ocean community answer about creating a swap file at droplet creation time.

So I decided to test how long using my old method (using dd) takes to run vs using fallocate.

Here’s how long it takes to run fallocate on a fresh 40GB droplet:

root@ubuntu:/# rm swapfile && time fallocate -l 1G /swapfile
real	0m0.003s
user	0m0.000s
sys	0m0.000s

root@ubuntu:/# rm swapfile && time fallocate -l 2G /swapfile
real	0m0.004s
user	0m0.000s
sys	0m0.000s

root@ubuntu:/# rm swapfile && time fallocate -l 4G /swapfile
real	0m0.006s
user	0m0.000s
sys	0m0.004s

root@ubuntu:/# rm swapfile && time fallocate -l 8G /swapfile
real	0m0.007s
user	0m0.000s
sys	0m0.004s

root@ubuntu:/# rm swapfile && time fallocate -l 16G /swapfile
real	0m0.012s
user	0m0.000s
sys	0m0.008s

root@ubuntu:/# rm swapfile && time fallocate -l 32G /swapfile
real	0m0.029s
user	0m0.000s
sys	0m0.020s

Interestingly, the relationship of size to time is non-linear when running fallocate.

Compare to building a 4GB swap file with dd (on the same server, it turned out using either a 16KB or 4KB bs gives the fastest run time):

time dd if=/dev/zero of=/swapfile bs=16384 count=262144 

262144+0 records in
262144+0 records out
4294967296 bytes (4.3 GB, 4.0 GiB) copied, 4.52602 s, 949 MB/s

real	0m4.528s
user	0m0.048s
sys	0m4.072s

Yes, you read that correctly – using dd with an “optimum” bs of 16KB (after much testing different bs settings) takes ~1000x as long as using fallocate to create the same “size” file!

How is fallocate so much faster? The details are in the man pages for it ^{_{(emphasis added)}}:

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 zeroes.

dd will “always” work. fallocate will work almostall of the time … but if you happen to be using a filesystem which doesn’t support it, you need to know how to use dd.

But: if your filesystem supports fallocate (and it probably does), it is orders of magnitude more efficient to use it for file creation.