fast-local-disks.md

---

layout: default
title: I/O-intensive computing
parent: 8. Working efficiently with data
grand_parent: Part 2
nav_order: 4
has_children: false
has_toc: false
permalink: /hands-on/data-io/io-exercise-fastdisks.html

---

How to run I/O intensive computing tasks efficiently?

Background

☝🏻 Lustre-based project-specific directories /scratch and /projappl can store large amounts of data and are accessible to all compute nodes of Puhti. However, these directories are not good for managing numerous files or performing intensive input/output (I/O) operations. If you need to work with a huge number of smaller files or perform frequent reads/writes, you should consider using the NVMe-based local temporary scratch directories, either through normal or interactive batch jobs.

💡 Read more about the advantages of using the local scratch disk in Docs CSC.

Convert the following regular batch job script into one that uses local scratch for faster I/O

💬 Below is a normal batch job script that pulls a docker image from DockerHub and converts it into an Apptainer image that is compatible with HPC environments such as CSC supercomputers Puhti and Mahti. During the conversion process, several layers are retrieved, cached and then converted into an Apptainer .sif image file.

1. Copy the script above to a file (e.g. batch_job.sh) and modify it accordingly.

   #!/bin/bash
   #SBATCH --account=<project>                          # Choose the billing project. Has to be defined!
   #SBATCH --time=01:00:00                              # Maximum duration of the job. Upper limit depends on the partition. 
   #SBATCH --partition=small                            # Job queues: test, interactive, small, large, longrun, hugemem, hugemem_longrun
   #SBATCH --mem=10G                                    # Reserve memory
   
   export APPTAINER_TMPDIR=/scratch/<project>/$USER     # Use these folders instead of the default $HOME
   export APPTAINER_CACHEDIR=/scratch/<project>/$USER   # $HOME has less space and you hate cleaning, don't you?
   
   apptainer pull --name trinity.simg docker://trinityrnaseq/trinityrnaseq
   

2. You can then submit the script file to a compute node using the command:

   sbatch batch_job.sh
   

💭 How long did it take to finish the job? What about when using NVMe?

Hints

• If you first ran the default script (above), then you need to clear the cache before running the next one.

• Request fast local storage using the --gres flag in the #SBATCH directive as follows:

  #SBATCH --gres=nvme:<local_storage_space_per_node_in_GB> 
  

• E.g., to request 200 GB of fast disk space, use:

  #SBATCH --gres=nvme:200
  

• Use the environment variable $LOCAL_SCRATCH to access the local storage on each compute node.

• Important! After you've processed the data on the fast local disk, remember to move it back to the shared disk area (/scratch), otherwise the data will be lost!

• Solution for script:

  #!/bin/bash
  #SBATCH --account=<project>                           # Choose the billing project. Has to be defined!
  #SBATCH --time=01:00:00                               # Maximum duration of the job. Upper limit depends on the partition. 
  #SBATCH --partition=small                             # Job queues: test, interactive, small, large, longrun, hugemem, hugemem_longrun
  #SBATCH --mem=10G                                     # Reserve memory
  #SBATCH --gres=nvme:100                               # Reservation of local NVMe storage. Default unit: GB
  
  export APPTAINER_TMPDIR=$LOCAL_SCRATCH                # Set the local storage area to the environment.. 
  export APPTAINER_CACHEDIR=$LOCAL_SCRATCH              # ..variable that Apptainer understands.
  unset XDG_RUNTIME_DIR                                 # Get rid of some unnecessary warnings in output
  
  cd $LOCAL_SCRATCH                                     # Move to the fast disk area
  pwd                                                   # Print the path
  apptainer pull --name trinity.simg docker://trinityrnaseq/trinityrnaseq
  mv trinity.simg /scratch/<project>/$USER/             # Move the output file back to /scratch
  

• Below is a comparison of execution time for running the same job on

• $LOCAL_SCRATCH vs. normal /scratch.

  	$LOCAL_SCRATCH	/scratch
  Wall-clock time	22m 06s	50m 06s