3Preparation
4Benchmark Execution

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Description

MDtest is an MPI-based application for evaluating the metadata performance of a file system and has been designed to test parallel file systems. MDTest is not a Lustre-specific benchmark and can be run on any POSIX-compliant file system, but it does require a fully installed and configured file system implementation in order to run. For Lustre, this means the MGS, MDS and OSS services must be installed, configured and running, and that there is a population of Lustre clients running with the Lustre file system mounted.

The mdtest application runs on Lustre clients in a fully configured Lustre file system. Multiple mdtest processes are run in parallel across several nodes using MPI in order to saturate file system I/O. The program can create directory trees of arbitrary depth and can be directed to create a mixture of work-loads, including file-only tests.

Purpose

MDTest measures the metadata performance of a given file system implementation and will run on any POSIX-compliant file system. The program works by creating, stat-ing and deleting a tree of directories and files in parallel across a population of machines (typically compute nodes in an HPC cluster). In the case of Lustre, the machines are Lustre clients. While mdtest can be run stand-alone to measure local file system performance, it is really intended to be run on parallel and shared file systems.

Metadata performance is a critical measurement of file system capability and is increasingly relevant to parallel file system workloads in general. It is therefore important to be able to demonstrate the ability of Lustre to match and even exceed application requirements for file systems. MDTest provides a way to define a standard test that can be used to assess baseline performance of a file system, and provide a comparative measure against storage platforms.

Preparation

The mdtest application is distributed as source code and must be compiled for use on the target environment. The preferred distribution of mdtest Torrent mtc pronest v10 crack. is available on GitHub at IOR as part of the IOR project. LANL have added features not available in the LLNL version, most notable of which are some Lustre-awareness to allow striping across multiple MDTs and AWS S3 support. There is indeed, even a third option, hidden in the depths of the Lustre JIRA issue tracking system in ticket LU-56 that adds the ability to run against multiple mountpoints on a single client.

The remainder of this document will use OpenMPI for the examples. Integration with job schedulers is not discussed – examples will call the mpirun command directly.

Download and Compile MDTest

To compile the mdtest binary, run the following steps on a suitable machine:

Install the pre-requisite development tools. On RHEL or CentOS systems, this can be accomplished by running the following command:
Download the mdtest source:
Compile the software:
Quickly verify that the program runs:
For example:
Copy the mdtest command onto all of the Lustre client nodes that will be used to run the benchmark. Alternatively, copy onto the Lustre file system itself so that the application is available on all of the nodes automatically.

Note: There is currently a bug in some versions of the libfabric library, notably version 1.3.0, that can cause a delay in starting MPI applications. When this occurs the following warning will appear in the command output:

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This issue affects RHEL and CentOS 7.3, and is resolved in RHEL / CentOS 7.4+ and the upstream project. Details can be found here:

Prepare the run-time environment

Create a user account from which to run the application, if a suitable account does not already exist. The account must be propagated across all of the Lustre client nodes that will participate in the benchmark, as well as the MDS servers for the file system. On the servers, it is recommended that the account is disabled in order to prevent users from logging into those machines.
Some MPI implementations rely upon passphrase-less SSH keys. Login as the benchmark user to one of the nodes and create a passphrase-less SSH key. This will enable the mpirun command to launch processes on each of the client nodes that will run the benchmark. For example:
Copy the public key into the $HOME/.ssh/authorized_keys file for the account.
If the user account is not hosted on a shared file system (e.g. a Lustre filesystem), then copy the public and private keys that were generated into the $HOME/.ssh directory of each of the Lustre client nodes that will be used in the benchmark. Normally, user accounts are hosted on a shared resource, making this step unnecessary.
Consider relaxing the StrictHostKeyChecking SSH option so that host entries are automatically added into $HOME/.ssh/known_hosts rather than prompting the user to confirm the connection. When running MPI programs across many nodes, this can save a good deal of inconvenience. If the account home directory is not on a shared storage, all nodes will need to be updated.
Install the MPI runtime onto all Lustre client nodes:
Append the following lines to $HOME/.bashrc (assuming BASH is the login shell) on the account running the benchmark:
This ensures that the Open MPI library path and binary path are added to the user environment every time the user logs in (and every time mpirun is invoked across multiple nodes). The .bash_profile file is not read when mpirun starts processes on remote nodes, which is why it is not chosen in this case.

Benchmark Execution

Login to one of the compute nodes as the benchmark user
Create a host file for the mpirun command, containing the list of Lustre clients that will be used for the benchmark. Each line in the file represents a machine and the number of slots (usually equal to the number of CPU cores). For example:
- The first column of the host file contains the name of the nodes. This can also be an IP address if the /etc/hosts file or DNS is not set up.
- The second column is used to represent the number of CPU cores.
Run a quick test using mpirun to launch the benchmark and verify that the environment is set up correctly. For example:
This should return the hostnames of all the machines that are in the test environment. The results are returned unsorted, in order of completion.
Note: If the --map-by node does not work, and the output has only one or a very small number of unique hostnames repeated in the output, then set slots=1 for each host in the host file. Otherwise, mpirun will fill up the slots on the first node before launching processes on subsequent nodes.
This may be desirable for multi-process tests but not for the single task per client test. Do not set the slot count higher than the number of cores present. If over-subscription is required, set the -np flag to greater than the number of physical cores. This informs OpenMPI that the task will be oversubscribed and will run in a mode that yields the processor to peers.
Refer to: OpenMPI FAQ -- Oversubscribing Nodes, and also the notes on OpenMPI at the

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