An I/O Analysis of HPC Workloads on CephFS and Lustre

In this contribution we compare the performance of the Input/Output load (I/O) of a High-Performance Computing (HPC) application on two different File Systems: CephFS and Lustre; our goal is to assess whether CephFS could be considered a valid choice for intense HPC applications. We perform our analysis using a real HPC workload, namely RegCM, a climate simulation application, and IOR, a synthetic benchmark application, to simulate several I/O patterns using different I/O parallel libraries (MPI-IO, HDF5, PnetCDF). We compare writing performance for the two different I/O approaches that RegCM implements: the so-called spokesperson or serial, and a truly parallel one. The small difference registered between the serial I/O approach and the parallel one motivates us to explore in detail how the software stack interacts with the underlying File Systems. For this reason, we use IOR and MP-IIO hints related to Collective Buffering and Data Sieving to analyze several I/O patterns on the two different File Systems. Finally we investigate Lazy I/O, a unique feature of CephFS, which disables file coherency locks introduced by the File System; this allows Ceph to buffer writes and to fully exploit its parallel and distributed architecture. Two clusters were set up for these benchmarks, one at CNR-IOM and a second one at Pawsey Supercomputing Centre; we performed similar tests on both installations, and we recorded a four-times I/O performance improvement with Lazy I/O enabled. Preliminary results collected so far are quite promising and further actions and new possible I/O optimizations are presented and discussed.