Data Transfer

What can slow down my data transfer?

To move data quickly, all of the stages in the process have to be fast; the file system you are reading data from, the CPU reading the data, the network connection between the sender and the reciever, and the recipient CPU and disk. The slowest element in that chain will slow down the entire transfer.

On SciNet's side, our underlying filesystem is the high-performance GPFS system, and the node we recommend you use (below) has a high-speed connection to the network and fast CPUs.

Data Mover Node

Serious moves of data to or from SciNet should be done from the datamover1 node. From any of the interactive SciNet nodes, one should be able to ssh datamover1 to log in. This is the machine that has the fastest network connection to the outside world (by a factor of 10; a 10Gb/s link as vs 1Gb/s).

Transfers must be originated from datamover1; that is, one can not copy files from the outside world directly to or from the data mover node; one has to log in to the data mover node and copy the data to or from the outside network.

hpn-ssh

All traffic to and from the data centre has to go via SSH, or secure shell. This is a protocol which sets up a secure connection between between two sites. On top of this protocol, there are many ways to copy files.

The usual ssh protocols were not designed for speed. On the datamover1 node, we have installed hpn-ssh, or High-Performance-enabled ssh. This is backwards compatable with the `usual' ssh, but is capable of significantly higher speeds. If you routinely have large data transfers to do, we recommend having your system administrator look into installing hpn-ssh on your system.

Everything we discuss below, unless otherwise stated, will work regardless of whether you have hpn-ssh installed on your remote system.

scp

scp, or secure copy, is the easiest way to copy files, although we generally find rsync below to be faster.

scp works like cp to copy files:

$ scp original_file  copy_file

except that either the original or the copy can be on another system:

$ scp jonsdatafile.bin jon@remote.system.com:/home/jon/bigdatadir/

will copy the data file into the directory /home/jon/bigdatadir/ on remote.system.com after logging in as jon; you will be prompted for a password (unless you've set up ssh keys).

Copying from remote systems works the same way:

$ scp jon@remote.system.com:/home/jon/bigdatadir/newdata.bin .

And wildcards work as you'd expect (except you have to quote the wildcards on the remote system, as it can't expand properly here.)

$ scp *.bin jon@remote.system.com:/home/jon/bigdatadir/
$ scp jon@remote.system.com:"/home/jon/inputdata/*" .

There are few options worth knowing about:

• scp -C compresses the file before transmitting it; if the file compresses well, this can significantly increase the effective data transfer rate (though usually not as much as compressing the data, then sending it, then uncompressing). If the file doesn't compress well, than this adds CPU overhead without accomplishing much, and can slow down your data transfer.
• scp -oNoneEnabled=yes -oNoneSwitch=yes -- if CPU overhead is a significant bottleneck in the data transfer, then we can remove some more by turning off the secure encryption of the data. For most of us, this is ok, but for others it is not. In either cases, authentication remains secure, it is only the data transfer that is in plaintext.

rsync

rsync is a very powerful tool for mirroring directories of data.

$ rsync -av -e ssh scinetdatadir jon@remote.system.com:/home/jon/bigdatadir/

rsync has a dizzying number of options; the above syncs scinetdatadir to the remote system; that is, any files that are newer on the localsysetm are updated on the remote system. The converse isn't true; if there were newer files on the remote system, you'd have to bring those over with

$ rsync -av -e ssh jon@remote.system.com:/home/jon/bigdatadir/ scinetdatadir 

The -av options are for verbose and `archive' mode, which preserves timestamps and permissions, which is normally what you want. -e ssh tells it to use ssh for the transfer.

One of the powerful things about rsync is that it looks to see what files already exist before copying, so you can use it repeatedly as a data directory fills and it won't make unnecessary copies; similarly, if a (say) log file grows over time, it will only copy the difference between the files, further speeding things up. This also means that it behaves well if a transfer is interrupted; a second invocation of rsync will continue where the other left off.

As with scp -C, rsync -z compresses on the fly, which can significantly enhance effective data transfer rates if the files compress well, or hurt it if not.

What transfer speeds could I expect?

Below are some typical transfer numbers from datamover1 to another University of Toronto machine with a 1Gb/s link to the campus network:

Mode	With hpn-ssh	Without
rsync	60-80 MB/s	30-40 MB/s
scp	50 MB/s	25 MB/s

Why are my transfers so much slower?

If you get numbers significantly lower than above, then there is a bottleneck in the transfer. The first thing to do is to run top on datamover1; if other people are transfering large files at the same time you are trying to, network congestion could result and you'll just have to wait until they are done.

If nothing else is going on on datamover1, there are a number of possibilites:

• network connection between SciNet and your machine - do you know the network connection of your remote machine? Are your systems connections tuned for performance [1]?
• is the remote server busy?
• are the remote servers disks busy, or known to be slow?

For any further questions, contact us at [2]