User Guide

This documentation is intended for users who need to run applications that make use of VeloC for checkpoint/restart.


Due to the large number of software and hardware configurations where VeloC can run, it must be built from source. Once built and installed, VeloC needs to be configured using a configuration file. These aspects are detailed below:

Download VeloC

The source code of VeloC is publicly available on github. To download it, look for the latest stable version x.y, which should appear under the ‘Releases/Tags’ tab as ‘veloc-x.y’. Then, use the following command:

git clone -b 'veloc-x.y' --single-branch --depth 1

If you want to experiment with the latest development version, you can directly check out the main branch. This is helpful to stay up-to-date with the latest features. The main branch is extensively tested and can be considered stable for most practical purposes.

git clone --single-branch --depth 1

Install VeloC

VeloC has an automated installation process based on Python, which depends on several standard libraries. These standard libraries may not be present on your system. If that is the case, you need to bootstrap the installation process first as follows:


Once the bootstrapping has finished, the script will build and install VeloC and all it dependencies. The installation can be fine-tuned with several options, which can be listed by supplying the “–help” switch. Notably, you can control the installation directory, communication protocol between the client and the backend, whether to use pre-installed libraries, etc. The script can be edited to modify certain compiler options if needed. Common compiler options needed for some machines (e.g. Cray) are included as comments. After editing the script, run it as follows:

$./ <install_dir>

Note that it may be possible that your Python installation will not detect the libraries installed by the bootstrapping automatically. In this case, the script will fail. Locate the installed libraries and tell Python about them as follows:

$setenv PYTHONPATH ~/.local/lib/python3.6/site-packages

If the installation process was successful, the VeloC client library (and its dependencies) are installed under <install_dir>/lib. The veloc.h header needed by the application developers to call the VeloC API is installed under <install_dir>/include. The active backend needed to run VeloC in asynchronous mode can be found in <install_dir/bin/veloc-backend. The examples can be found in <source_dir>/src/test, while the corresponding compiled executables are here: <source_dir>/build/test.

Configure VeloC

VeloC uses a INI-style configuration with the following mandatory fields:

scratch = <path> (node-local path where VELOC can save temporary checkpoints that live for the duration of the reservation)
persistent = <path> (persistent path where VELOC can save durable checkpoints that live indefinitely)

In addition, the following optional fields are available:

persistent_interval = <int> (seconds between consecutive persistent checkpoints, default: 0 - perform all)
ec_interval = <int> (seconds between consecutive EC checkpoints, default: 0 - perform all)
watchdog_interval = <int> (seconds between consecutive checks of client processes: default: 0 - don't check)
max_versions = <int> (number of previous checkpoints to keep on persistent, default: 0 - keep all)
scratch_versions = <int> (number of previous checkpoints to keep on scratch, default: 0 - keep all)
failure_domain = <string> (failure domain used for smart distribution of erasure codes, default: <hostname>)
axl_type = <string> (AXL read/write strategy to/from the persistent path, default: <empty> - deactivate AXL)
chksum = <boolean> (activates checksum calculation and verification for checkpoints, default: false)
meta = <path> (persistent path where VELOC will save checksumming information)

Both the persistent and ec interval can be set to -1, which fully deactivates that feature. This is preferred to setting a high number (which also works but is less readable and has slightly higher overhead because VeloC will need to do extra checks). If you leave scratch_versions to the default value, you must ensure the scratch mount point will run out of space. VELOC will not automatically delete checkpoints when space is low. If space is a concern, set scratch_versions accordingly. Similar observations apply for the persistent mount point, for which the corresponding option is max_versions. Finally, you can specify whether to use a built-in POSIX file transfer routine to flush the files to a parallel file system or to use the AXL library for optimized flushes that can take advantage of additional hardware to accelerate I/O (such as burst buffers). If the use of AXL is desired, you need to specify as axl_type as per the AXL documentation (which is part of VELOC). Note that VELOC uses a separate meta path for checksumming information, instead of writing checksumming information directly into the checkpoints. Thus, it is perfectly valid to save checksumming information during checkpointing but then delete or ignore it later on restart (in which case the meta option must be omitted).


VeloC can be run in either synchronous mode (all resilience strategies are embedded in the client library and run directly in the application processes in blocking fashion) or asynchronous mode (the resilience strategies run in a separate process called the active backend in asynchronous mode in the background).

To use VeloC in synchronous mode, the application simply needs to be run as any normal MPI job. To run VeloC in asynchronous mode, you need to make sure the veloc-backend executable can either be found in the $PATH or $VELOC_BIN environment variable. This is true for every node running the MPI ranks of your application (thus, veloc-backend should be accessible through a shared mount point). By default, veloc-backend will create the following log file on each node: /dev/shm/veloc-backend-<host_name>-<uid>.log. The log file contains important information (error messages, time to flush to PFS, etc.) that you may want to collect and inspect while/after running your application. In this case, you can control where the log files are saved using $VELOC_LOG environment variable (e.g., a shared directory).


VeloC comes with a series of examples in the test subdirectory that can be used to test the setup. To run these examples (in either synchronous or asynchronous mode), edit the sample configuration file heatdis.cfg and then run the application as follows (run the active backend first as mentioned above if in async mode):

mpirun -np N <source_dir>/build/test/heatdis_mem <mem_per_process> <config_file>

Batch Jobs

HPC machines are typically configured to run the user applications as batch jobs. Therefore, the user needs to make sure that the job scheduler is not configured to kill the entire job when a node fails. Assuming the job scheduler is configured correctly, the user needs to write a script as follows:

reserve N+K nodes (to survive a maximum of K total failures over the entire application runtime)
    run the application (on the surviving nodes)
while (failure detected) // e.g, exit code of the application