Storage performance: Lustre file system

Betzy, Fram and Stallo

To get best throughput on the scratch file system (/cluster/work), you may need to change the data striping. Striping shall be adjusted based on the client access pattern to optimally load the object storage targets (OSTs). On Lustre, the OSTs are referring to disks or storage volumes constructing the whole file system.

The stripe_count indicates how many OSTs to use. The stripe_size indicates how much data to write to one OST before moving to the next OST.

  • Striping will only take affect only on new files, created or copied into the specified directory or file name.
  • Default stripe_count on /cluster file system on Fram and Stallo is 1.
  • Betzy is implementing Progressive File Layouts to dynamically set file stripe size based on file size growth.

Betzy: Progressive File Layouts

PFL removes the need to explicitly specify striping for each file, assigning different Lustre striping characteristics to contiguous segments of a file as it grows. Dynamic striping allows lower overhead for small files and assures increased bandwidth for larger files. However, note that for workloads with significant random read phases it is best to manually assign stripe size and count.

  • Betzy implements another new feature, called data on metadata for small files with size under 2KB.

Betzy: Data on Metadata

Lustre file system performance is optimized for large files. To balance that, data on metadata (DoM) is enabled on Betzy to ensure higher performance in case of frequently accessed small files. Files accessed with a size of 2KB or smaller will be stored on a very fast NVMe JBOD directly connected to the metadata servers.

For more detailed information on striping, please consult the Lustre documentation.

How to find out the current striping

To see current stripe size (in bytes), use lfs getsripe [file_system, dir, file] command. e.g.:

$ lfs getstripe /cluster/tmp/test

/cluster/tmp/test
stripe_count:   1 stripe_size:    1048576 stripe_offset:  -1

Rules of thumb for proper stripe counts

For best performance we urge you to always profile the I/O characterstics of your HPC application and tune the I/O behavior.

Down below is a list of rules you may apply to properly set stripe count for your files:

  • files smaller than 1GB: default striping;
  • files size between 1GB - 10GB: stripe count 2;
  • files size between 10GB - 1TB: stripe count 4;
  • files bigger than 1TB: stripe count 8.

Large files

For large files it is advisable to increase stripe count and perhaps chunk size too. e.g.:

# stripe huge file across 8 OSTs
$ lfs setstripe --stripe-count 8 "my_file"

# stripe across 4 OSTs using 8 MB chunks.
$ lfs setstripe --stripe-size 8M --stripe-count 4 "my_dir"

It is advisable to use higher stripe count for scientific application that writes to a single file from hundreds of nodes, or a binary executable that is loaded by many nodes when an application starts.

Choose a stripe size between 1 MB and 4 MB for sequential I/O. Larger than 4MB stripe size may result in performance loss in case of shared files.

Set the stripe size a multiple of the write() size, if your application is writing in a consistent and aligned way.

Small files

For many small files and one client accessing each file, change stripe count to 1. Avoid having small files with large stripe counts. This negatively impacts the performance due to the unnecessary communication to multiple OSTs.

$ lfs setstripe --stripe-count 1 "my_dir"

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