CernVM-FS on Supercomputers

There are several characteristics in which supercomputers can differ from other nodes with respect to CernVM-FS

  1. Fuse is not allowed on the individual nodes
  2. Individual nodes do not have Internet connectivity
  3. Nodes have no local hard disk to store the CernVM-FS cache

These problems can be overcome as described in the following sections.

Parrot-Mounted CernVM-FS in lieu of Fuse Module

Instead of accessing /cvmfs through a Fuse module, processes can use the Parrot connector. The parrot connector works on x86_64 Linux if the ptrace system call is not disabled. In contrast to a plain copy of a CernVM-FS repository to a shared file system, this approach has the following advantages:

  • Millions of synchronized meta-data operations per node (path lookups, in particular) will not drown the shared cluster file system but resolve locally in the parrot-cvmfs clients.
  • The file system is always consistent; applications never see half-synchronized directories.
  • After initial preloading, only change sets need to be transfered to the shared file system. This is much faster than rsync, which always has to browse the entire name space.
  • Identical files are internally de-duplicated. While space of the order of terabytes is usually not an issue for HPC shared file systems, file system caches benefit from deduplication. It is also possible to preload only specific parts of a repository namespace.
  • Support for extra functionality implemented by CernVM-FS such as versioning and variant symlinks (symlinks resolved according to environment variables).

Preloading the CernVM-FS Cache

The cvmfs_preload utility can be used to preload a CernVM-FS cache onto the shared cluster file system. Internally it uses the same code that is used to replicate between CernVM-FS stratum 0 and stratum 1. The cvmfs_preload command is a self-extracting binary with no further dependencies and should work on a majority of x86_64 Linux hosts.

The cvmfs_preload command replicates from a stratum 0 (not from a stratum 1). Because this induces significant load on the source server, stratum 0 administrators should be informed before using their server as a source. As an example, in order to preload the ALICE repository into /shared/cache, one could run from a login node

cvmfs_preload -u http://cvmfs-stratum-zero-hpc.cern.ch:8000/cvmfs/alice.cern.ch -r /shared/cache

This will preload the entire repository. In order to preload only specific parts of the namespace, you can create a _dirtab_ file with path prefixes. The path prefixes must not involve symbolic links. An example dirtab file for ALICE could look like

/example/etc
/example/x86_64-2.6-gnu-4.8.3/Modules
/example/x86_64-2.6-gnu-4.8.3/Packages/GEANT3
/example/x86_64-2.6-gnu-4.8.3/Packages/ROOT
/example/x86_64-2.6-gnu-4.8.3/Packages/gcc
/example/x86_64-2.6-gnu-4.8.3/Packages/AliRoot/v5*

The corresponding invokation of cvmfs_preload is

cvmfs_preload -u http://cvmfs-stratum-zero-hpc.cern.ch:8000/cvmfs/alice.cern.ch -r /shared/cache \
  -d </path/to/dirtab>

The initial preloading can take several hours to a few days. Subsequent invokations of the same command only transfer a change set and typically finish within seconds or minutes. These subsequent invokations need to be either done manually when necessary or scheduled for instance with a cron job.

The cvmfs_preload command can preload files from multiple repositories into the same cache directory.

Access from the Nodes

In order to access a preloaded cache from the nodes, set the path to the directory as an Alien Cache. Since there won’t be cache misses, parrot or fuse clients do not need to download additional files from the network.

If clients do have network access, they might find a repository version online that is newer than the preloaded version in the cache. This results in conflicts with cvmfs_preload or in errors if the cache directory is read-only. Therefore, we recommend to explicitly disable network access for the parrot process on the nodes, for instance by setting

HTTP_PROXY='INVALID-PROXY'

before the invocation of parrot_run.

Compiling cvmfs_preload from Sources

In order to compile cvmfs_preload from sources, use the -DBUILD_PRELOADER=yes cmake option.

Loopback File Systems for Nodes’ Caches

If nodes have Internet access but no local hard disk, it is preferable to provide the CernVM-FS caches as loopback file systems on the cluster file system. This way, CernVM-FS automatically populates the cache with the latest upstream content. A Fuse mounted CernVM-FS will also automatically manage the cache quota.

This approach requires a separate file for every node (not every mountpoint) on the cluster file system. The file should be 15% larger than the configured CernVM-FS cache size on the nodes, and it should be formatted with an ext3/4 or an xfs file system. These files can be created with the dd and mkfs utilities. Nodes can mount these files as loopback file systems from the shared file system.

Because there is only a single file for every node, the parallelism of the cluster file system can be exploited and all the requests from CernVM-FS circumvent the cluster file system’s meta-data server(s).