Note - As of July 1st, 2016, files on scratch filesystems are subject to deletion 60 days after they were created.
User Scratch Space
Each compute node has its own local scratch filesystem. Users may read from and write to this using their own exclusive directory at /localscratch/Users/<HawkID>.
In addition to local storage, each HPC cluster has its own large, shared filesystem mounted across all its nodes via NFS. Analogously, users can read from and write to this using their own exclusive directory at /nfsscratch/Users/<HawkID>.
Cleanup Policy
Scratch filesystems are a shared resource available for the convenience of all users. Therefore, files on these filesystems are subject to deletion after a certain lifespan as specified by the HPC policy committee. As of July 1 2016, the allowed file lifespan is 60 days after first being written. On /nfsscratch, a file's age is the time elapsed since its creation timestamp ("crtime"), which is tracked on the fileserver. An automated cleanup process will run periodically on the server to delete files whose crtime has reached the maximum lifespan.
Home account storage and purchased storage are *not* subject to this policy.
Duplicating files to circumvent the scratch cleanup process is against policy. Please move your results to stable storage in a timely manner.
Please contact hpc-sysadmins@iowa.uiowa.edu if you need assistance with this.
File Timestamps
Note that crtime is distinct from the other timestamps on a file:
- modification time (mtime): This is the time the contents of the file were last modified, for example, by editing it. The modification time can be seen with
ls -l file
- change time (ctime): This is the time the metadata of the file was last changed. An example of this would be moving a file to a different directory. The change time can be seen with
ls -lc file
- access time (atime): This is the time the contents of the file were last accessed; for example, by viewing with 'less'. The access time can be seen with
ls -lu file
- creation time (crtime): This is the time the contents of the file were first written to the filesystem. This attribute is part of the underlying ZFS filesystem and is not accessible via NFS or standard Linux utilities.
It is possible for all of these timestamps to be different for a single file. Most archive utilities will maintain the first 3 timestamps, either by default or optionally. This includes using archive mode ('-a') with either 'cp' or 'rsync'. However, note that no utility can affect a file's crtime at all over NFS.
Local or Shared Scratch?
- Multiple jobs might be running on your job's node. These jobs can compete for local storage I/O, causing contention independent of /nfsscratch. Only a job with exclusive access to a node can expect the full performance potential of the node's local storage.
- A parallel job running on multiple nodes typically shouldn't use filesystems local to any of its nodes. Even if you're writing your own MPI instead of using an off-the-shelf application, you can expect better performance if you collate results in memory via message passing and write your result to the shared filesystem. Consider local disk primarily as a structured alternative to swap.
- If your job places partial results on /localscratch but fails to handle them for any reason (logic error, eviction, crash, etc.), you won't have access to these anywhere else and they will be difficult to recover.
- As always, please test a few jobs if you are unsure.