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Descriptions of the different filesystems for data storage on COSMOS and tips for efficient I/O performance.

Home Spaces

Mounted on /nfs/home. Every user has a quota on the space and the number of files in their home space.

CXFS Storage

  • /fast/space

This should be where your files generated by your applications go. It may be automatically periodically cleaned of old files, so after filtering out the important files and data, it should be moved elsewhere.

  • /fast/flagship

Area where certain projects have their own disc space.

  • /slow/space

This is where your filtered, important files and frequently used input files should reside in. There is a per-project quota of 1 TB unless otherwise agreed with the PI. Note that there is a lot more space available than 1 TB / project, so the limit of 1 TB is simply there to ensure usage does not get out of control and we are happy to increase it for good reasons while there is more space available.

  • /slow/data

It has been replaced by /slow/space. As the underlying hardware is out of maintenance no new data should be put there.

Parallel I/O

CXFS although not a 'proper' parallel one like Lustre or GPFS on a cluster, is still capable of parallel i/o operations. A few rules need to be followed to achieve the maximum throughput -

  1. The number of concurrent I/O operations should be kept low - 2-8 is the best choice, although up to 16 could be ok - depends on the file sizes. It means that large MPI applications which sometime allow each MPI process to perform I/O will suffer if left with the default settings. Gadget2/3 - is an example of the sensible approach, which allows the user to choose the number of parallel i/o streams.
  2. Only parallel access to different files is efficient - access to one file cannot be accomplished in parallel, unless the file is relatively small to fit into the system cache.
  3. As is a common situation on many HPC systems, Cosmos stores are optimised for the operations on large files - it is always better to keep the data in a few large ones, rather in a many small ones. Optimal file size is in the range of 100KB - 100GB.