Change-HI/EDU

File Transfers with Remote Computers

Computing with a remote computer offers very limited use if we cannot get files to or from the cluster. There are several options for transferring data between computing resources, from command line options to GUI programs, which we will cover here.

Download Files From the Internet

One of the most straightforward ways to download files is to use either curl or wget, one of these is usually installed in most Linux shells, on Mac OS terminal and in GitBash. Any file that can be downloaded in your web browser through a direct link can be downloaded using curl -O or wget. This is a quick way to download datasets or source code.

The syntax for these commands is: curl -O https://some/link/to/a/file and wget https://some/link/to/a/file. Try it out by downloading some material we’ll use later on, from a terminal on your local machine.

Transferring Single Files and Folders With scp

To copy a single file to or from the cluster, we can use scp (“secure copy”). The syntax can be a little complex for new users, but we’ll break it down.

To upload to another computer:

To download from another computer:

Note that everything after the : is relative to our home directory on the remote computer. We can leave it at that if we don’t care where the file goes.

To copy a whole directory, we add the -r flag, for “recursive”: copy the item specified, and every item below it, and every item below those… until it reaches the bottom of the directory tree rooted at the folder name you provided.

[user@laptop ~]$ scp -r some-local-folder yourUsername@hpc-dtn1.its.hawaii.edu:target-directory/

What’s in a /?

When using scp, you may have noticed that a : always follows the remote computer name; sometimes a / follows that, and sometimes not, and sometimes there’s a final /. On Linux computers, / is the root directory, the location where the entire filesystem (and others attached to it) is anchored. A path starting with a / is called absolute, since there can be nothing above the root /. A path that does not start with / is called relative, since it is not anchored to the root.

If you want to upload a file to a location inside your home directory — which is often the case — then you don’t need a leading /. After the :, start writing the sequence of folders that lead to the final storage location for the file or, as mentioned above, provide nothing if your home directory is the destination.

A trailing slash on the target directory is optional, and has no effect for scp -r, but is important in other commands, like rsync.

A Note on rsync

As you gain experience with transferring files, you may find the scp command limiting. The rsync utility provides advanced features for file transfer and is typically faster compared to both scp and sftp (see below). It is especially useful for transferring large and/or many files and creating synced backup folders.

The syntax is similar to scp. To transfer to another computer with commonly used options:

The a (archive) option preserves file timestamps and permissions among other things; the v (verbose) option gives verbose output to help monitor the transfer; the z (compression) option compresses the file during transit to reduce size and transfer time; and the P (partial/progress) option preserves partially transferred files in case of an interruption and also displays the progress of the transfer.

To recursively copy a directory, we can use the same options:

As written, this will place the local directory and its contents under the specified directory on the remote system. If the trailing slash is omitted on the destination, a new directory corresponding to the transferred directory (‘dir’ in the example) will not be created, and the contents of the source directory will be copied directly into the destination directory.

The a (archive) option implies recursion.

To download a file, we simply change the source and destination:

A Note on Ports

All file transfers using the above methods use SSH to encrypt data sent through the network. So, if you can connect via SSH, you will be able to transfer files. By default, SSH uses network port 22. If a custom SSH port is in use, you will have to specify it using the appropriate flag, often -p, -P, or --port. Check --help or the man page if you’re unsure.

Transferring Files Interactively with FileZilla

FileZilla is a cross-platform client for downloading and uploading files to and from a remote computer. It is absolutely fool-proof and always works quite well. It uses the sftp protocol. You can read more about using the sftp protocol in the command line …

Download and install the FileZilla client from https://filezilla-project.org. After installing and opening the program, you should end up with a window with a file browser of your local system on the left hand side of the screen. When you connect to the cluster, your cluster files will appear on the right hand side.

To connect to the cluster, we’ll just need to enter our credentials at the top of the screen:

Hit “Quickconnect” to connect. You should see your remote files appear on the right hand side of the screen. You can drag-and-drop files between the left (local) and right (remote) sides of the screen to transfer files.

Finally, if you need to move large files (typically larger than a gigabyte) from one remote computer to another remote computer, SSH in to the computer hosting the files and use scp or rsync to transfer over to the other. This will be more efficient than using FileZilla (or related applications) that would copy from the source to your local machine, then to the destination machine.

Archiving Files

One of the biggest challenges we often face when transferring data between remote HPC systems is that of large numbers of files. There is an overhead to transferring each individual file and when we are transferring large numbers of files these overheads combine to slow down our transfers to a large degree.

The solution to this problem is to archive multiple files into smaller numbers of larger files before we transfer the data to improve our transfer efficiency. Sometimes we will combine archiving with compression to reduce the amount of data we have to transfer and so speed up the transfer.

The most common archiving command you will use on a (Linux) HPC cluster is tar. tar can be used to combine files into a single archive file and, optionally, compress it.

Let’s start with the file we downloaded from the lesson site, hpc-intro-data.tar.gz. The “gz” part stands for gzip, which is a compression library. Reading this file name, it appears somebody took a folder named “hpc-intro-data,” wrapped up all its contents in a single file with tar, then compressed that archive with gzip to save space. Let’s check using tar with the -t flag, which prints the “table of contents” without unpacking the file, specified by -f <filename>, on the remote computer. Note that you can concatenate the two flags, instead of writing -t -f separately.

This shows a folder containing another folder, which contains a bunch of files. Let’s see about that compression, using du for “disk usage”.

Files Occupy at Least One “Block”

If the filesystem block size is larger than 36 KB, you’ll see a larger number: files cannot be smaller than one block.

Now let’s unpack the archive. We’ll run tar with a few common flags:

When it’s done, check the directory size with du and compare.

If you want to reverse the process — compressing raw data instead of extracting it — set a c flag instead of x, set the archive filename, then provide a directory to compress:

Working with Windows

When you transfer text files to from a Windows system to a Unix system (Mac, Linux, BSD, Solaris, etc.) this can cause problems. Windows encodes its files slightly different than Unix, and adds an extra character to every line.

On a Unix system, every line in a file ends with a \n (newline). On Windows, every line in a file ends with a \r\n (carriage return + newline). This causes problems sometimes.

Though most modern programming languages and software handles this correctly, in some rare instances, you may run into an issue. The solution is to convert a file from Windows to Unix encoding with the dos2unix command.

You can identify if a file has Windows line endings with cat -A filename. A file with Windows line endings will have ^M$ at the end of every line. A file with Unix line endings will have $ at the end of a line.

To convert the file, just run dos2unix filename. (Conversely, to convert back to Windows format, you can run unix2dos filename.)