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Durham University


Frequently asked questions

Here is a list of frequently asked, or otherwise useful questions. If you have a question you'd like to see here, please let us know. For COSMA8-specific questions, please see the COSMA8 FAQ

How do I best formulate my questions to the support staff?

Please direct questions to Explain what you are trying to do or would like to do, where you are trying to do it, and provide your COSMA username.

Who should I consult for technical problems? Which mailing list should I use?

For technical problems, please email If we cannot solve your problems, we will then try to find people who can.

Who can get an account on cosma?

Any STFC theory community member, or any ICC collaborator. To request DiRAC funded time, either submit to the standard call for proposals, or submit a seedcorn application. See the DiRAC website for further details. For non-DiRAC time, speak to your collaborator at Durham, CC-ing

What should I do to get an account?

Follow these instructions.

Can my external collaborator get an account too?

Yes, provided that they are a collaborator with the ICC

How do I log in to cosma

Follow these instructions. This will work both from within and outside Durham University.

Can I use vnc? Where should I run the server?

VNC is not recommended for security reasons. However, x2go offers a faster graphical interface.

What are the main differences between cosma5, cosma6, cosma7 and cosma8?

Please see here.

How do I decide which COSMA to use?

If you are working on a DiRAC project, you should use COSMA6, COSMA7 or COSMA8, depending on which project you are working. You need to be a member of the relevant Operating System Group (use the "id" command on a login node), to submit to the corresponding COSMA.

If you are not part of a DiRAC project, you should use COSMA5. You will need to be part of the "durham" group to do this.

How do I copy (large) files to/from cosma?

Please see here.

We have a Globus Online endpoint, cosma#data. Please create an account, and use this.

For intermediate sized files, you can always use scp or rsync.

Where is my home directory?

/cosma/home/PROJECT/USERNAME (where PROJECT will be durham or a DiRAC project identifier)

You will also have data storage space, typically in one of:

  • /cosma5/data/PROJECT/USERNAME
  • /cosma6/data/PROJECT/USERNAME
  • /cosma7/data/PROJECT/USERNAME
  • /cosma8/data/PROJECT/USERNAME

Are there disk usage quotas?

Yes, these should be set. You have a quota both on total storage capacity (typically 10TB for data and 10GB for homespace), and total number of files used.

How do I check my quota?

On a login node, use the "quota" command. This should report all your quotas on the different file systems (/cosma5, /cosma6, /cosma7 and homespace). This is part of the cosma module, so you may need to "module load cosma" first.

Alternatively, on the appropriate login node, use the c5quota (which no longer works), c6quota, or c7quota commands. You will need the cosma module loaded. To see your homespace quota, use the "quota" command.

Is anything backed-up?

Homespace is backed up every few days. Data space can be archived to tape media upon request.

Can I print from cosma?

This is not currently possible, though if you are in Durham, you can email a PDF to the CIS printers using the "mail" command.

What are these “modules”?

Modules are used to setup your work environment, changing your environment variables (e.g. PATH, etc) depending on what your requirements are.

Which modules should I load? Are these different on the different cosma’s? How do I find which modules are available?

Use the "modules available" command to see available modules. These are the same for each COSMA system, and if optimised versions exist for particular architectures, these may be loaded automatically.

What are the recommended modules for running Gadget? Arepo?

Please see the codes section of the site for specific code details.

How do I adapt the Makefile to be consistent with these modules?

Well written Makefiles should just work. However, badly written Makefiles may need changing to make use of the environment variables created by loading modules. The "export" command will show you the currently set environment variables.

How do I load modules automatically on login?

Put "module load MODULE" commands within your .bash_profile script (or .login if you are an older user)

I want to perform a large simulation: Who do I ask? Can I just run?

It is recommended to speak to first, so that we are aware of your requirements. We may also be able to make recommendations.

How do I write the submission script?

Follow these instructions.

What are the main commands to interact with a batch job?

See here.

How do I make sure my job runs on full nodes?

COSMA nodes are exclusive - you will not share nodes with other users. However, this does mean you should make good use of all the available cores.

If you are using a single core job, consider running multiple of these at the same time, using SLURM arrays, or a parallel MPI launch.

How many cores can I reasonably request?

COSMA5 has about 5000

COSMA6 has about 9000 cores.

COSMA7 has 12,656 cores.

COSMA8 has 46,080 cores.

What determines when my job starts? How do I know the job is running? How can I kill it?

The SLURM job submission system has a fair scheduler, based on how many jobs you have previously submitted, how busy the queues are, and how large your job is. If you specify a job with a shorter run-time, it is more likely to be scheduled more quickly to fill in space.

For job control, please see here.

Where should I put the data? Is that backed-up?

You will have space in /cosma[5,6,7,8]/data/PROJECT/USERNAME

This data is not backed up, but operates on a maintained parallel storage system (i.e. individual disk failures do not lead to data loss), and is archived periodically.

Each storage location is optimally connected to that system. e.g. if you store data in /cosma6/ please access it from the cosma6 queue. It will not be available to cosma5 or cosma7 compute nodes.

Do I need to worry about where the initial conditions are stored?

Yes. These should be stored on the data storage connected to the compute nodes you are using. e.g. for cosma6, use /cosma6/data/

If initial conditions are small and only used by a small number of nodes at a time, they may be better stored in your home space.

How do I run an embarrassingly parallel job?

Investigate using SLURM arrays

Can I log-on to a compute node? Should I?

You can generate ssh keys on a COSMA login node, and copy the public part to your .ssh/authorized_keys file. This will then alow you to ssh directly to a compute node. However, unless you have good reasons for doing this, please don't.

How do I find how busy the system is?

SLURM commands such as squeue and sview will tell you this.

What is “back-fill”? How do I exploit this?

If a large (many node) job is waiting to run, it cannot start until enough nodes are available. This therefore means that some nodes become idle, and can be used as "back-fill" for short jobs. To use this, submit your job as usual, and if the system deems it appropriate, it will schedule you quickly. To make best use of this, specify a short time period for your job.

The c[5,6,7,8]backfill command (e.g. c5backfill) will show available nodes for back-filling.

I want to analyse a simulation, but need a lot of memory: how do I do this?

COSMA5 and COSMA6 login nodes have 512GB, while COSMA7 login nodes have 1.5TB memory. COSMA8 login nodes have 2TB RAM. This is shared between other logged in users, so please check for a quiet node first.

The mad01 system has 3TB memory.

The mad02 system has 1.5TB RAM.

The mad03 system has 6TB RAM (using Apache Pass non-volatile memory, so performance is somewhat reduced).

The mad04 system has 4TB RAM.

I want to visualise my data: what can I use?

Standard visualisation tools are available. If you need an answer to this question, please ask us to put more details here. Several GPU servers are also available.

I am developing a code and need to profile and debug it

What are the debugging tools?

allinear, gdb, perf, etc are all installed.

What are the profiling tools?

allinear, gdb, perf, etc are all installed

My short tests take a long time to start: can I do something about this?

Specify a short time period in you batch script file.

Can I run interactively on a node? How do I do this?

The srun command will allow you to do this.

Tell me about parallel compile. Can I compile on cosma6 and then run on cosma7?

To compile in parallel, use the -j NNN argument when you type make (e.g. make -j 4 to use 4 threads during compilation).

Code compiled on COSMA5 or COSMA6 will run on COSMA7 and COSMA8. Heavily optimised code compiled on COSMA7 may not run on COSMA5 or COSMA6, as these have older CPU architectures.

Does mounting cosma via sshfs on your local machine hog any cosma resources?

If you begin moving large files about, you may notice poor performance.

How do I find out the disk quota I have? How can I get more?

With the cosma module loaded ("module load cosma"), use the "quota" command.

To increase your quota, please email

My jobs die without creating an output

Probably, you are trying to access storage not belonging to that particular COSMA. e.g. from the cosma7 queue, you are trying to write to /cosma5. This is not allowed (inefficient for large jobs). So, you need to make sure that all reading/writing is done to locally attached storage.

I am in many groups, how do I change my default one?

The "newgrp" command can be used to do this. If the "id" command shows you being a member of several groups, you can change between them using "newgrp GROUPNAME". This will only take effect for your current shell. However, it can be used within batch scripts if you wish to write your data as a particular group.

I cannot write to my data space in /cosma*/data/PROJECT/USERNAME. Please help...

First, check that you have write premission on this directory:

ls -ld /path/to/directory

This should show something like drwxr-xr-x 1234 USERNAME GROUP ..., which means that you have read, write and execute permission, that members of the group have read and execute permission, and that everyone else has read and execute permission. If the w for write permission is not present (e.g. dr-xr-xr-x) you should add it: chmod u+w /path/to/directory

Check that the directory is owned by you, i.e. that the USERNAME returned from the ls -ld command is your username!

If that is all okay, check that you have sufficient quota left to write data there: "module load cosma" and then "quota". This should be done on a login node (login5, login6 or login7).

Finally, if you are still not able to write, check that the group you are writing as still has quota left. To identify your group, use the "id" command. This will show gid=XXXX(group) where XXXX is a number. You can then use e.g. c5quota -g XXXX (or c6quota or c7quota). Also, please note that your default group may not be that used to write your data, if your directory has a "sticky" bit set. In this case, the "ls -ld" command will show something like drwxr-sr-x. Note the "s". You will be writing as the specified group into this directory, regardless of what your default group is. Then check that this group is not over quota.

To change the group that you are writing as, if a sticky bit is set, either remove the sticky bit (chmod g-s /path/to/directory) or re-group the directory (chown USER:NEWGROUP /path/to/directory). If a sticky bit is not set, and your default group is over quota, so you wish to write as a different group, you can use the "newgrp" command: "newgrp GROUP". This will change your group to that specified (which should be in the list given by the "id" command), for the current terminal. Therefore, you will need to do this again if you log in again. You will also need to put it into your Slurm batch scripts. Alternatively, if you believe that your default group is wrong, and wish it to be changed, please contact cosma-support.

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