So I decided to test how long using my old method (using
dd) takes to run vs using
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
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!
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
fallocatesystem 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
But: if your filesystem supports
fallocate (and it probably does), it is orders of magnitude more efficient to use it for file creation.