Walking Randomly

The High Performance Computing system at University of Sheffield has several different file systems available to it. We have:-

• /fastdata – A lustre-based, shared filesystem with hundreds of terabytes of space. No backup. No quota.
• /data – An NFS file system where each user has access to 100Gb of storage. Back-ups go back 7 days.
• /home –  An NFS file system where each user has 10Gb. Backed up over 28 days. Mirrored.
• /scratch – Local disk on each worker node. No back up. Uses ext4.

Lots of options with differing amounts of space, back-up policy and, as I’m about to demonstrate, performance characteristics. I suspect that many other HPC systems have a similar set up.

On our system, it’s very tempting to do everything in /fastdata. There’s lots of space, no quota, readable from all worker nodes simultaneously — good times! I try to encourage people to think about what they are doing, however. Bad things can happen if the lustre filesystem is hammered too much. Also, there can be a huge difference in performance for some operations across different filesystems.

Let’s take an example. I want to download and untar gcc 4.9.2. How long does that take on the three different filesystems?

On the scratch directory of a worker node

cd \scratch
mkdir testing123
cd testing123
wget ftp://ftp.mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-4.9.2/gcc-4.9.2.tar.gz
time tar xfz ./gcc-4.9.2.tar.gz 

real    0m6.237s
user    0m5.302s
sys 0m3.033s

On the lustre filesystem

cd /fastdata/
mkdir testing123
cd testing123
wget ftp://ftp.mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-4.9.2/gcc-4.9.2.tar.gz
time tar xfz ./gcc-4.9.2.tar.gz 

real    7m18.170s
user    0m6.751s
sys 0m56.802s

On the NFS filesystem

cd /data/myusername
mkdir testing123
cd testing123
wget ftp://ftp.mirrorservice.org/sites/sourceware.org/pub/gcc/releases/gcc-4.9.2/gcc-4.9.2.tar.gz
time tar xfz ./gcc-4.9.2.tar.gz 

real	16m37.343s
user	0m6.052s
sys	0m23.438s

For this particular operation, there is a two orders of magnitude difference between the worst and the best option.

I’m not an expert in filesystems and I have no idea what’s causing these differences or if I’d see a similar speed difference given a different file operation. I currently have no interest in doing a robust set of benchmarks. The point I’m making is that if you are using a system that has multiple filesystems it may be worth checking if there’s an advantage to using one over the other for your particular use case.