Magnus users do not receive a space quota-ed allocation on the scratch file system. Instead the file system is treated as a shared resource on which older files are removed periodically to free up space. In general, files are considered to be old -- and thus candidates for deletion -- if they have not been accessed (either read from or written to) for 30 DAYS.
Please note that from 2019 we will be imposing a quota on the number of files/directories each user will be allowed to store on /scratch of 1 million. If you have more than this, you may find that jobs will fail when writing output.
You can check your usage of /scratch at any time by running lfs quota /scratch which will show you (first line, '14' in the example below) how many files you have on /scratch
aelwell@magnus-1:~$ lfs quota /scratch
Disk quotas for user aelwell (uid 20701):
Filesystem kbytes quota limit grace files quota limit grace
/scratch 1460 0 0 - 14 0 0 -
/scratch is a shared resource that suffers low performance when the number of files increases too much. USERS SHOULD REMOVE THEIR FILES THEMSELVES if those files are no longer in use, rather than leaving them for purge. For removing a large number of files check: Deleting large numbers of files on scratch and group