Data storage and transfer

Overview

In this lesson, we will learn the basics of data storage and transfer on OSG Connect.

Stash

OSG Connect provides a storage system called Stash. Stash provides a place to store data needed for jobs or output from jobs in the medium term. Since Stash is not backed up, you should transfer job outputs from Stash to your local system as soon as practical.

This lesson will go over accessing Stash using the OSG Connect login node as well as other methods such as Globus, and HTTP.

Exploring the Stash system

First, we'll look at accessing Stash from the login node. You'll need to log in to OSG Connect:

ssh username@login.osgconnect.net #Connect to the login node with your username
passwd:       # your password

Once done, you can change to the stash directory in your home area:

$ cd ~/stash    

This directory is an area on Stash that you can use to store files and directories. It functions just like any other UNIX directory although it has additional functions that we'll go over shortly.

For future use, let's create a file in Stash:

$ cd ~/stash
$ echo "Hello world" > my_hello_world

Transferring files to and from Stash using SCP

We can transfer files to Stash using scp. First, let's look at transferring files using scp. Scp is a counterpart to ssh that allows for secure, encrypted file transfers between systems using your ssh credentials.

To transfer a file from Stash using scp, you'll need to run scp with the source and destination. Files on remote systems are indicated using user@machine:/path/to/file . Let's copy the file we just created from Stash to our local system:

$ scp username@login.osgconnect.net:~/data/my_hello_world .

As you can see, scp uses similar syntax to the cp command that you were shown previously. To copy directories using scp, you'll just pass the -r option to it. E.g:

$ scp -r username@login.osgconnect.net:~/data/my-directory .

Challenges

• Create a directory with a file called hello_world_2 in the ~/data directory and copy it from Stash to your local system.
• Create a directory called hello_world_3 on your local system and copy it to the data directory.

Transferring files to and from Stash using Globus

An alternate method for accessing Stash is to use Globus. Globus allows you to initiate transfers between Globus endpoints and will handle the actual file and directory transfers transparently without needing further input from you. When the transfer is complete, Globus will send a notification to you indicating this.

Let's transfer a file from your laptop to Globus. Doing this will require an application called Globus Connect Personal that allows Globus to transfer files and directories to and from your laptop. First go to the Globus page and download and install the globus connect personal installer specific to your system.

While that's running, you'll need to get a setup key from Globus in order to setup the Globus Connect Personal software.

• First login to OSG Connect using your OSG Connect username and password
• Next go to this page
• Click on the add Globus Connect Personal link
• Enter a name for your endpoint on the page (remember this!)
• Click on "Generate setup Key"
• Select the key and copy the key

Finally, start the Globus online personal software that you just installed. The installer will ask for the setup key that you obtained from the Globus website. At this point, the install and setup of Globus Connect Personal is complete.

Now go to http://www.osgconnect.net and under the Transfer menu, select Start Transfer. For the first endpoint, enter username#name where name is the name you choose for the endpoint above. You should now see the files from your laptop displayed. For the second endpoint, enter osgconnect#stash and hit enter. You should now see the contents of your home directory on OSG Connect. Now double click on the data directory. Select a file on your laptop and click on the right arrow on the top of the screen to start a transfer to Stash. You can transfer files or directories to your laptop by selecting it in the Stash window and selecting the left arrow.

Challenges

• Copy a file to Stash from your laptop using Globus.
• Next copy the my_hello_world file from Stash to your laptop using Globus.

Transferring files from Stash using HTTP

Stash also allows you to access files using your web browser. In order to do this, you'll need to put your file in ~/publicor ~/data/public (the two locations point to the same directory). Any file or directory that is placed here will be made available in the Stash webserver. Let's make a file available using the Stash webserver

$ cd ~/public
$ echo "This is served over the web" > web-file

Now go to http://stash.osgconnect.net/+username/ in your browser. You should see the file in the listing. Clicking on the file should give you the contents.

Challenge

• Create a file called my-web-file and make it available through the Stash webserver.

Using data on Stash in compute jobs

Let us do an example calculation to understand the use of Stash and how we download the data from the web. We will peform a molecular dynamics simulation of a small protein in implicit water. To get the necessary files, we use the tutorial command on OSG.

Log in to OSG Connect:

$ ssh username@login.osgconnect.net

Type:

$ tutorial stash-namd
$ cd ~/tutorial-stash-namd

Aside: NAMD is a widely used molecular dynamics simulation program. It lets users specify a molecule in some initial state and then observe its time evolution subject to forces. Essentially, it lets you go from a specifed molecular structure to a simulation of its behavior in a particular environment. It has been used to study polio eradication, similations of graphene, and studies of biofuels.

You should see the following files in the directory:

$ ls
namd_stash_run.submit #HTCondor job submission script file.
namd_stash_run.sh #Job execution script file.
ubq_gbis_eq.conf #Input configuration for NAMD.
ubq.pdb #Input pdb file for NAMD.
ubq.psf #Input file for NAMD.
par_all27_prot_lipid.inp #Parameter file for NAMD.

The file par_all27_prot_lipid.inp is the parameter file and is required for the NAMD simulations. The parameter file is common data file for the NAMD simulations. It is a good practice to keep the common files, like the parameter file in our example, in the Stash storage.

mv par_all27_prot_lipid.inp ~/public/.  

You can view the parameter file using your web browser by going to http://stash.osgconnect.net/+yourusername.

Now we want the parameter file available on the execution (worker) machine when the simulation starts to run. As mentioned early, the data on the Stash is available to the execution machines. This means the execution machine can transfer the data from Stash as a part of the job execution. So we have to script this in the job execution script.

You can see that the job script namd_stash_run.sh has the following lines:

$ cat namd_stash_run.sh
#!/bin/bash 
source /cvmfs/oasis.opensciencegrid.org/osg/modules/lmod/5.6.2/init/bash 
module load namd/2.9  
wget http://stash.osgconnect.net/+username/par_all27_prot_lipid.inp  
namd2 ubq_gbis_eq.conf  

In the above script, you will have to insert your "username" in URL address. The parameter file located on Stash is downloaded using the #wget# utility.

Now we submit the NAMD job.

$ condor_submit namd_stash_run.submit 

Once the job completes, you will see non-empty "job.out.0" file where the standout output from the programs are written as default.

$ tail job.out.0

WallClock: 6.084453  CPUTime: 6.084453  Memory: 53.500000 MB
Program finished.

The above lines indicate the NAMD simulation was successful.