Filestores¶

Every HPC user has access to up to five different storage areas:

Home directories: per-user backed-up, snapshotted storage
Data directories: additional per-user snapshotted storage (not on Bessemer)
Fastdata areas: high-performance shared filesystem for temporary data - optimised for reading/writing large files from multiple nodes and threads simultaneously
Shared (project) directories: per-PI shared storage areas (snapshotted and backed-up) for project data - can be accessed from non-HPC machines too
Scratch directories: per-node temporary storage - useful for reading/writing lots of small files within one job

The storage areas differ in terms of:

the amount of space available;
whether they are available from multiple nodes;
whether they are shared between clusters;
whether the underlying storage system is performant if reading/writing large files;
whether the underlying storage system is performant if reading/writing small files;
frequency of storage snapshotting, whether storage is mirrored and the maximum duration data can be retained for.

Home directories¶

All users have a home directory on each system, some of which are shared between systems:

System	Path	Type	Quota per user	Shared between system login and worker nodes?	Shared between systems?
Bessemer	`/home/yourusername`	NFS	100GB	Yes	No
ShARC	`/home/yourusername`	NFS	10GB	Yes	ShARC + Iceberg
Iceberg	`/home/yourusername`	NFS	10GB	Yes	ShARC + Iceberg

Snapshotting and mirrored backups¶

Frequency of snapshotting	Snapshots retained	Backed up onto separate storage system
Every 4 hours	10 most recent	Yes
Every night	Last 28 days	Yes

Data directories¶

Every user on Iceberg and ShARC (not Bessemer) has access to a larger data storage area:

System	Path	Type	Quota per user	Shared between system login and worker nodes?	Shared between systems?
Bessemer	N/A	NFS	N/A	N/A	N/A
ShARC	`/data/yourusername`	NFS	100GB	Yes	ShARC + Iceberg
Iceberg	`/data/yourusername`	NFS	100GB	Yes	ShARC + Iceberg

Snapshotting and mirrored backups¶

Frequency of snapshotting	Snapshots retained	Backed up onto separate storage system
Every 4 hours	10 most recent	No
Every night	Last 7 days	No

Automounting¶

Data directories are made available to you (mounted) on demand: if you list the contents of just /data after first logging on then /data/yourusername subdirectories might not be shown. However, if you list the contents of /data/yourusername itself or change into that directory then its contents will appear.

Later on if you list the contents of /data again you may find that /data/yourusername has disappeared again, as it is automatically unmounted following a period of inactivity.

Fastdata areas¶

Fastdata areas are optimised for large file operations. These areas are Lustre filesystems.

They are are faster than Home directories, Data directories and Shared (project) directories when dealing with larger files but are not performant when reading/writing lots of small files (Scratch directories are ideal for reading/writing lots of small temporary files within jobs). An example of how slow it can be for large numbers of small files is detailed here.

System	Path	Type	Quota per user	Filesystem capacity	Shared between systems?	Network bandwith per link
Bessemer	`/fastdata`	Lustre	None	460 TB	No	25Gb/s Ethernet
ShARC	`/fastdata`	Lustre	None	669 TB	ShARC’s fastdata filesystem is accessible from Iceberg	100Gb/s (Omni-Path)
Iceberg	`/fastdata-sharc`	Lustre	None	669 TB	ShARC’s fastdata filesystem is accessible from Iceberg	1Gb/s Ethernet

Snapshotting and mirrored backups¶

Snapshotting is not enabled for fastdata areas and these areas are not backed up.

Managing your files in fastdata areas¶

In order to avoid interference from other users’ files it is important that you store your files in a directory created and named the same as your username. e.g.

mkdir /fastdata/yourusername

By default the directory you create will have world-read access. If you want to restrict read access to just your account then run

chmod 700 /fastdata/yourusername

after creating the directory. A more sophisticated sharing scheme would have private and public directories

mkdir /fastdata/yourusername
mkdir /fastdata/yourusername/public
mkdir /fastdata/yourusername/private

chmod 755 /fastdata/yourusername
chmod 755 /fastdata/yourusername/public
chmod 700 /fastdata/yourusername/private

Automatic file deletion¶

Warning

There are no quota controls in fastdata areas but older files are automatically deleted: a report of files older than 60 days is regularly generated, the owners of these files are then notified by email then a week after the email(s) are sent the identified files are deleted.

We reserve the right to change this policy without warning in order to ensure efficient running of the service.

It is important to therefore not use fastdata areas for long-term storage and copy important data from these areas to areas suitable for longer-term storage (Home directories, Data directories (not backed up) or Shared (project) directories).

You can use the lfs command to find out which files in a fastdata directory are older than a certain number of days and hence approaching the time of deletion. For example, to find files 50 or more days old

lfs find -ctime +50 /fastdata/yourusername

File locking¶

POSIX file locking is not enabled on these Lustre filesystems, which can cause issues for certain applications that require/expect it (e.g. programs that create/use SQLite databases).

Shared (project) directories¶

Each PI at the University is entitled to request a free 10 TB storage area for sharing data with their group and collaborators. The capacity per area can be extended and additional shared areas can be purchased (both at a cost).

After one of these project storage areas has been requested/purchased it can be accessed in two ways:

as a Windows-style (SMB) file share on machines other than ShARC/Iceberg using \\uosfstore.shef.ac.uk\shared\;
as a subdirectory of /shared on ShARC/Iceberg (you need to explicitly request HPC access when you order storage from IT Services).

Snapshotting and mirrored backups¶

Frequency of snapshotting	Snapshots retained	Backed up onto separate storage system
Every 4 hours	10 most recent	Yes
Every night	Last 7 days	Yes

Automounting¶

Similar to Data directories, subdirectories beneath /shared are mounted on demand on the HPC systems: they may not be visible if you simply list the contents of the /shared directory but will be accessible if you cd (change directory) into a subdirectory e.g. cd /shared/my_group_file_share1.

Specifics for Bessemer¶

If you need to access a /shared area on Bessemer please contact helpdesk@sheffield.ac.uk to arrange this.

Warning

If you access a /shared directory stored in Sheffield from Bessemer then you may experience slower performance, espeicially for small files.
Network traffic between Bessemer and Sheffield Research Filestore is not encrypted when travelling between Sheffield and Leeds over JANET
/shared areas can be created on Bessemer’s filestore system if you need faster access from Bessemer

Permissions behaviour¶

You may encounter strange permissions issues when running programs on HPC against the /shared areas e.g. chmod +x /shared/mygroup1/myprogram.sh fails. Here we try to explain why.

Behind the scenes, the file server that provides this shared storage manages permissions using Windows-style ACLs (which can be set by area owners via the Research Storage management web interface. However, the filesystem is mounted on a Linux cluster using NFSv4 so the file server therefore requires a means for mapping Windows-style permissions to Linux ones. An effect of this is that the Linux mode bits for files/directories under /shared on the HPC systems are not always to be believed: the output of ls -l somefile.sh may indicate that a file is readable/writable/executable when the ACLs are what really determine access permissions. Most applications have robust ways of checking for properties such as executability but some applications can cause problems when accessing files/directories on /shared by naively checking permissions just using Linux mode bits:

which: a directory under /shared may be on your path and you may be able to run a contained executable without prefixing it with a absolute/relative directory but which may fail to find that executable.
Perl: scripts that check for executability of files on /shared using -x may fail unless Perl is explicitly told to test for file permissions in a more thorough way (see the mention of use filetest 'access' here).
git: may complain that permissions have changed if a repository is simply moved to /shared/someplace from elsewhere on Bessemer/ShARC/Iceberg. As a workaround you can tell git to not to track Linux permissions for a single repository using git config core.filemode false or for all repositories using git config --global core.filemode false.

Changing how attempts to change permissions are handled: each /shared area can be configured so that

Attempts to change file/directory mode bits fail (e.g. chmod +x /shared/mygroup1/myprogram.sh fails) (default configuration per area) or
Attempts to change file/directory mode bits appear to succeed (e.g. chmod +x /shared/mygroup1/myprogram.sh does not fail but also does not actually change any permissions on the underlying file server) (alternative configuration per area)

If you would like to switch to using the second way of handling permissions for a particular /shared/ area then the Owner of this area should make a request via the Helpdesk.

Further information¶

The documentation for the /shared storage service includes information on:

how access/permissions are managed
how to create folders with associated permissions within /shared storage areas

Scratch directories¶

For jobs that need to read/write lots of small files the most performant storage will be the temporary storage on each node (under the /scratch directory).

This is because with Home directories, Data directories, Fastdata areas, Shared (project) directories each time a file is accessed the filesystem needs to request ownership/permissions information from another server and for small files these overheads are proportionally high. However, for /scratch such ownership/permissions metadata is available on the local machine, so it is faster when dealing with small files.

The most obvious disadvantage to the /scratch node-local storage is that a given directory cannot reliabily be accessed between jobs as you cannot guarantee that your next job will run on the same node. Any data of value must therefore be copied off /scratch (e.g. to Home directories or Data directories) before the end of your job.

Where to store data beneath /scratch: The scheduler automatically creates a per-job directory for you under /scratch. If you started your job using qrshx, qsh or qsub then the name of this directory is stored in the $TMPDIR environment variable e.g.

[[email protected] ~]$ qrshx
[[email protected] ~]$ cd $TMPDIR
[[email protected] 667443.1.all.q]$ pwd
/scratch/667443.1.all.q

The scheduler will then clean up (delete) $TMPDIR at the end of your job, ensuring that the space can be used by other users.

If using qrsh to start your job then the environment variable will unfortunately be undefined so you will need to manually create a directory under /scratch (named using your username) and this will not be cleaned up when the job ends.

Anything under the /scratch may be deleted periodically when the worker-node is idle. /scratch is not backed up. There are no quotas for /scratch storage.

/scratch uses the ext4 filesystem.

How to check your quota usage¶

To find out your storage quota usage for your home directory, data directory (if not on Bessemer) and particular Shared (project) directories:

df -h somedirectoryname

For example:

Storage area	Command to check quota
Home directory	`df -h /home/$USER`
Data directory	`df -h /data/$USER`
A Shared (project) directory	`df -h /shared/myproject1`

If you exceed your filesystem quota¶

If you reach your quota for your home directory then many common programs/commands may cease to work as expected or at all and you may not be able to log in.

In addition, jobs may fail if you exceed your quota for your data directory or a Shared (project) directory.

In order to avoid this situation it is strongly recommended that you:

Check your quota usage regularly.
Copy files that do not need to be backed up to a Fastdata area or remove them from Bessemer/ShARC/Iceberg completely.

Recovering files from snapshots¶

Home directories, Data directories and Shared (project) directories are regularly snapshotted. See above for details of the snapshot schedules per area. A subset of snapshots can be accessed by HPC users from the HPC systems themselves by explicitly browsing to hidden directories e.g.

Storage area	Parent directory of snapshots
Home directory	`/home/$USER/.snapshot`
Data directory	`/data/$USER/.snapshot`
A Shared (project) directory	`/shared/myproject1/.snapshot`

From within per-snapshot directories you can access (read-only) copies of files/directories. This allows you to attempt recover any files you might have accidentally modified or deleted recently.

Note that .snapshot directories are not visible when listing all hidden items within their parent directories (e.g. using ls -a /home/$USER): you need to explicitly cd into .snapshot directories to see/access them.