Storage Overview

OLCF users have many options for data storage. Each user has multiple user-affiliated storage spaces, and each project has multiple project-affiliated storage spaces where data can be shared for collaboration. Below we give an overview and explain where each storage area is mounted.

Storage Areas

The storage area to use in any given situation depends upon the activity you wish to carry out. Each user has a User Home area on a Network File System (NFS) and a User Archive area on the archival High Performance Storage System (HPSS). These user storage areas are intended to house user-specific files. Each project has a Project Home area on NFS, multiple Project Work areas on Spectrum Scale, and a Project Archive area on HPSS. These project storage areas are intended to house project-centric files. We have defined several areas as listed below by function:

  • User Home: Long-term data for routine access that is unrelated to a project. It is mounted on compute nodes of Summit as read only
  • User Archive: Long-term data for archival access that is unrelated to a project.
  • Project Home: Long-term project data for routine access that’s shared with other project members. It is mounted on compute nodes of Summit as read only
  • Member Work: Short-term user data for fast, batch-job access that is not shared with other project members.
  • Project Work: Short-term project data for fast, batch-job access that’s shared with other project members.
  • World Work: Short-term project data for fast, batch-job access that’s shared with OLCF users outside your project.
  • Project Archive: Long-term project data for archival access that’s shared with other project members.

Alpine IBM Spectrum Scale Filesystem

Summit mounts a POSIX-based IBM Spectrum Scale parallel filesystem called Alpine. Alpine’s maximum capacity is 250 PB. It is consisted of 77 IBM Elastic Storage Server (ESS) GL4 nodes running IBM Spectrum Scale 5.x which are called Network Shared Disk (NSD) servers. Each IBM ESS GL4 node, is a scalable storage unit (SSU), constituted by two dual-socket IBM POWER9 storage servers, and a 4X EDR InfiniBand network for up to 100Gbit/sec of networking bandwidth. The maximum performance of the final production system will be about 2.5 TB/s for sequential I/O and 2.2 TB/s for random I/O under FPP mode, which means each process, writes its own file. Metada operations are improved with around to minimum 50,000 file access per sec and aggregated up to 2.6 million accesses of 32KB small files.

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Figure 1. An example of the NDS servers on Summit

Performance under not ideal workload

The I/O performance can be lower than the optimal one when you save one single shared file with non-optimal I/O pattern. Moreover, the previous performance results are achieved under an ideal system, the system is dedicated, and a specific number of compute nodes are used. The file system is shared across many users; the I/O performance can vary because other users that perform heavy I/O as also executing large scale jobs and stress the interconnection network. Finally, if the I/O pattern is not aligned, then the I/O performance can be significantly lower than the ideal one. Similar, related to the number of the concurrent users, is applied for the metadata operations, they can be lower than the expected performance.

Tips

  • For best performance on the IBM Spectrum Scale filesystem, use large page aligned I/O and asynchronous reads and writes. The filesystem blocksize is 16MB, the minimum fragment size is 16K so when a file under 16K is stored, it will still use 16K of the disk. Writing files of 16 MB or larger, will achieve better performance. All files are striped across LUNs which are distributed across all IO servers.

  • If your application occupies up to two compute nodes and it requires a significant number of I/O operations, you could try to add the following flag in your job script file and investigate if the total execution time is decreased. This flag could cause worse results, it depends on the application.

    #BSUB -alloc_flags maximizegpfs

Major difference between Lustre and IBM Spectrum Scale

The file systems have many technical differences, but we will mention only what a user needs to be familiar with:

  • On Summit, there is no concept of striping from the user point of view, the user uses the Alpine storage without the need to declare the striping for files/directories. The GPFS will handle the workload, the file system was tuned during the installation.