ABSTRACT

PLFS: A Checkpoint Filesystem for Parallel Applications

LANL Technical Release LA-UR 09-02117.

John Bent*, Garth Gibson†, Gary Grider*, Ben McClelland*, Paul Nowoczynski‡, James Nunez*, Milo Polte†,
Meghan Wingate*

*Los Alamos National Laboratory
†Carnegie Mellon University
‡Pittsburgh Supercomputing Center

Parallel applications running across thousands of processors must protect themselves from inevitable com-
ponent failures. Many applications insulate themselves from failures by checkpointing, a process in which they
save their state to persistent storage. Following a failure, they can resume computation using this state. For
many applications, saving this state into a shared single file is most convenient. With such an approach, the size
of writes are often small and not aligned with file system boundaries. Unfortunately for these applications, this
preferred data layout results in pathologically poor performance from the underlying file system which is optimized
for large, aligned writes to non-shared files.

To address this fundamental mismatch, we have developed a parallel log-structured file system, PLFS, which
is positioned between the applications and the underlying parallel file system. PLFS remaps an application’s
write access pattern to be optimized for the underlying file system. Through testing on Panasas ActiveScale
Storage System and IBM’s General Parallel File System at Los Alamos National Lab and on Lustre at Pittsburgh
Supercomputer Center, we have seen that this layer of indirection and reorganization can reduce checkpoint time
by up to several orders of magnitude for several important benchmarks and real applications.

We expect that PLFS can improve the checkpoint bandwidth for any large parallel application that writes to
a single file. The expected improvement is especially large for those applications doing unaligned or random IO,
patterns which have become increasingly prevalent recently due to the wide-spread adoption of complex formatting
libraries such as NetCDF and HDF5.