Admin Guide Lustre Tuning

This article describes tuning options for the Lustre file system with the example of the Noctua cluster of PC².

Lustre File System

The high-performance parallel file system of Noctua is a Lustre File System.

This Lustre file system has three major functional units

• one Metadata Server (MDS) with two metadata targets (MDTs)
  • stores namespace metadata, such as filenames, directories, Access permissions, and file layout
  • stores small files on SSDs to accelerate data access
• two Object Storage Servers (OSS), each with two Object Storage Targests (OST)
  • each OST manages a single local disc filesystem
• Clients (the Noctua nodes) that access the data (read/write)
  • Lustre presents all Clients with a unified Namespace for all the files and data in the file System
  • allows concurrent and coherent read and write access to the files in the filesystem

Key points

• Lustre achieves high Performance through parallelism
  • best Performance from multiple Clients writing to multiple OSTs
• Lustre is designed to achieve high bandwidth to/from a small number of files
  • used as a scratch file System
  • good match for scientific datasets and/or checkpoint data
• Lustre is not designed to handle large numbers of small files
  • potential bottle necks at the MDS when files are opened
  • data will not be spread over multiple OSTs
  • not a good choice for program compilation

A powerful Lustre utility is lfs. The tool has a built-in help system.

> lfs help
Available commands are:
        setstripe
        getstripe
        setdirstripe
        getdirstripe
        mkdir
        rm_entry
        pool_list
        find
...

Metadata operations are expensive

• the stat operations return information on file ownerships, permissions, size, update times etc.
• to obtain the file size requires a lookup on the MDS and an enquiry for file size on each OST owning a stripe

therefore

• avoid ls -l (like color ls)
• avoid file completion in shells
• open and fail instead of stat/INQUIRE
• don't stripe small files, Lustre check every OST that might own a part of the file
• open a file read-only if that is what you will do
• use tools optimzied for (aware of) Lustre
  • lfs find, lfs df, ...
  • stripe-aware tar (star)
• avoid to read the same files on many processes, better to read on one process and use MPI communication to move data to other processes
• avoid large directories, organize directory structure by processes/clients
• open() and seek() if you know the size, otherwise try to organize applications to write from only one process
• use the Lustre API in your application (see man lustreapi)

More Tuning tips for Lustre are in Noctua Tuning.