CDC File Transfer

Born from the ashes of Stadia, this repository contains tools for syncing and streaming files from Windows to Windows or Linux. The tools are based on Content Defined Chunking (CDC), in particular FastCDC, to split up files into chunks.

History

At Stadia, game developers had access to Linux cloud instances to run games. Most developers wrote their games on Windows, though. Therefore, they needed a way to make them available on the remote Linux instance.

As developers had SSH access to those instances, they could use scp to copy the game content. However, this was impractical, especially with the shift to working from home during the pandemic with sub-par internet connections. scp always copies full files, there is no "delta mode" to copy only the things that changed, it is slow for many small files, and there is no fast compression.

To help this situation, we developed two tools, cdc_rsync and cdc_stream, which enable developers to quickly iterate on their games without repeatedly incurring the cost of transmitting dozens of GBs.

CDC RSync

cdc_rsync is a tool to sync files from a Windows machine to a Linux device, similar to the standard Linux rsync. It is basically a copy tool, but optimized for the case where there is already an old version of the files available in the target directory.

cdc_rsync demo

The remote diffing algorithm is based on CDC. In our tests, it is up to 30x faster than the one used in rsync (1500 MB/s vs 50 MB/s).

The following chart shows a comparison of cdc_rsync and Linux rsync running under Cygwin on Windows. The test data consists of 58 development builds of some game provided to us for evaluation purposes. The builds are 40-45 GB large. For this experiment, we uploaded the first build, then synced the second build with each of the two tools and measured the time. For example, syncing from build 1 to build 2 took 210 seconds with the Cygwin rsync, but only 75 seconds with cdc_rsync. The three outliers are probably feature drops from another development branch, where the delta was much higher. Overall, cdc_rsync syncs files about 3 times faster than Cygwin rsync.

Comparison of cdc_rsync and Linux rsync running in Cygwin

We also ran the experiment with the native Linux rsync, i.e syncing Linux to Linux, to rule out issues with Cygwin. Linux rsync performed on average 35% worse than Cygwin rsync, which can be attributed to CPU differences. We did not include it in the figure because of this, but you can find it here.

How does it work and why is it faster?

The standard Linux rsync splits a file into fixed-size chunks of typically several KB.

Linux rsync uses fixed size chunks

If the file is modified in the middle, e.g. by inserting xxxx after 567, this usually means that the modified chunks as well as all subsequent chunks change.

Fixed size chunks after inserting data

The standard rsync algorithm hashes the chunks of the remote "old" file and sends the hashes to the local device. The local device then figures out which parts of the "new" file matches known chunks.

Syncing a file with the standard Linux rsync
Standard rsync algorithm

This is a simplification. The actual algorithm is more complicated and uses two hashes, a weak rolling hash and a strong hash, see here for a great overview. What makes rsync relatively slow is the "no match" situation where the rolling hash does not match any remote hash, and the algorithm has to roll the hash forward and perform a hash map lookup for each byte. rsync goes to great lengths optimizing lookups.

cdc_rsync does not use fixed-size chunks, but instead variable-size, content-defined chunks. That means, chunk boundaries are determined by the local content of the file, in practice a 64 byte sliding window. For more details, see the FastCDC paper or take a look at our implementation.

cdc_rsync uses variable, content-defined size chunks

If the file is modified in the middle, only the modified chunks, but not subsequent chunks change (unless they are less than 64 bytes away from the modifications).

Content-defined chunks after inserting data

Computing the chunk boundaries is cheap and involves only a left-shift, a memory lookup, an add and an and operation for each input byte. This is cheaper than the hash map lookup for the standard rsync algorithm.

Because of this, the cdc_rsync algorithm is faster than the standard rsync. It is also simpler. Since chunk boundaries move along with insertions or deletions, the task to match local and remote hashes is a trivial set difference operation. It does not involve a per-byte hash map lookup.

Syncing a file with cdc_rsync
cdc_rsync algorithm

CDC Stream

cdc_stream is a tool to stream files and directories from a Windows machine to a Linux device. Conceptually, it is similar to sshfs, but it is optimized for read speed.

To efficiently determine which parts of a file changed, the tool uses the same CDC-based diffing algorithm as cdc_rsync. Changes to Windows files are almost immediately reflected on Linux, with a delay of roughly (0.5s + 0.7s x total size of changed files in GB).

cdc_stream demo

The tool does not support writing files back from Linux to Windows; the Linux directory is readonly.

The following chart compares times from starting a game to reaching the menu. In one case, the game is streamed via sshfs, in the other case we use cdc_stream. Overall, we see a 2x to 5x speedup.

Comparison of cdc_stream and sshfs

Supported Platforms

cdc_rsync From To
Windows x86_64 ✓ ✓ 1
Ubuntu 22.04 x86_64 ✗ 2 ✓
Ubuntu 22.04 aarch64 ✗ ✗
macOS 13 x86_64 3 ✗ ✗
macOS 13 aarch64 3 ✗ ✗
cdc_stream From To
Windows x86_64 ✓ ✗
Ubuntu 22.04 x86_64 ✗ ✓
Ubuntu 22.04 aarch64 ✗ ✗
macOS 13 x86_64 3 ✗ ✗
macOS 13 aarch64 3 ✗ ✗

1 Only local syncs, e.g. cdc_rsync C:\src\* C:\dst. Support for remote syncs is being added, see #61.
2 See #56.
3 See #62.

Getting Started

Download the precompiled binaries from the latest release to a Windows device and unzip them. The Linux binaries are automatically deployed to ~/.cache/cdc-file-transfer by the Windows tools. There is no need to manually deploy them. We currently provide Linux binaries compiled on Github's latest Ubuntu version. If the binaries work for you, you can skip the following two sections.

Alternatively, the project can be built from source. Some binaries have to be built on Windows, some on Linux.

Prerequisites for Building

To build the tools from source, the following steps have to be executed on both Windows and Linux.

Finally, install an SSH client on the Windows machine if not present. The file transfer tools require ssh.exe and sftp.exe.

Building

The two tools CDC RSync and CDC Stream can be built and used independently.

CDC RSync

CDC Stream

Usage

The tools require a setup where you can use SSH and SFTP from the Windows machine to the Linux device without entering a password, e.g. by using key-based authentication.

Configuring SSH and SFTP

By default, the tools search ssh.exe and sftp.exe from the path environment variable. If you can run the following commands in a Windows cmd without entering your password, you are all set:

ssh user@linux.device.com
sftp user@linux.device.com

Here, user is the Linux user and linux.device.com is the Linux host to SSH into or copy the file to.

If additional arguments are required, it is recommended to provide an SSH config file. By default, both ssh.exe and sftp.exe use the file at %USERPROFILE%\.ssh\config on Windows, if it exists. A possible config file that sets a username, a port, an identity file and a known host file could look as follows:

Host linux_device
	HostName linux.device.com
	User user
	Port 12345
	IdentityFile C:\path\to\id_rsa
	UserKnownHostsFile C:\path\to\known_hosts

If ssh.exe or sftp.exe cannot be found, you can specify the full paths via the command line arguments --ssh-command and --sftp-command for cdc_rsync and cdc_stream start (see below), or set the environment variables CDC_SSH_COMMAND and CDC_SFTP_COMMAND, e.g.

set CDC_SSH_COMMAND="C:\path with space\to\ssh.exe"
set CDC_SFTP_COMMAND="C:\path with space\to\sftp.exe"

Note that you can also specify SSH configuration via the environment variables instead of using a config file:

set CDC_SSH_COMMAND=C:\path\to\ssh.exe -p 12345 -i C:\path\to\id_rsa -oUserKnownHostsFile=C:\path\to\known_hosts
set CDC_SFTP_COMMAND=C:\path\to\sftp.exe -P 12345 -i C:\path\to\id_rsa -oUserKnownHostsFile=C:\path\to\known_hosts

Note the small -p for ssh.exe and the capital -P for sftp.exe.

Google Specific

For Google internal usage, set the following environment variables to enable SSH authentication using a Google security key:

set CDC_SSH_COMMAND=C:\gnubby\bin\ssh.exe
set CDC_SFTP_COMMAND=C:\gnubby\bin\sftp.exe

Note that you will have to touch the security key multiple times during the first run. Subsequent runs only require a single touch.

CDC RSync

cdc_rsync is used similar to scp or the Linux rsync command. To sync a single Windows file C:\path\to\file.txt to the home directory ~ on the Linux device linux.device.com, run

cdc_rsync C:\path\to\file.txt user@linux.device.com:~

cdc_rsync understands the usual Windows wildcards * and ?.

cdc_rsync C:\path\to\*.txt user@linux.device.com:~

To sync the contents of the Windows directory C:\path\to\assets recursively to ~/assets on the Linux device, run

cdc_rsync C:\path\to\assets\* user@linux.device.com:~/assets -r

To get per file progress, add -v:

cdc_rsync C:\path\to\assets\* user@linux.device.com:~/assets -vr

The tool also supports local syncs:

cdc_rsync C:\path\to\assets\* C:\path\to\destination -vr

CDC Stream

To stream the Windows directory C:\path\to\assets to ~/assets on the Linux device, run

cdc_stream start C:\path\to\assets user@linux.device.com:~/assets

This makes all files and directories in C:\path\to\assets available on ~/assets immediately, as if it were a local copy. However, data is streamed from Windows to Linux as files are accessed.

To stop the streaming session, enter

cdc_stream stop user@linux.device.com:~/assets

The command also accepts wildcards. For instance,

cdc_stream stop user@*:*

stops all existing streaming sessions for the given user.

Troubleshooting

On first run, cdc_stream starts a background service, which does all the work. The cdc_stream start and cdc_stream stop commands are just RPC clients that talk to the service.

The service logs to %APPDATA%\cdc-file-transfer\logs by default. The logs are useful to investigate issues with asset streaming. To pass custom arguments, or to debug the service, create a JSON config file at %APPDATA%\cdc-file-transfer\cdc_stream.json with command line flags. For instance,

{ "verbosity":3 }

instructs the service to log debug messages. Try cdc_stream start-service -h for a list of available flags. Alternatively, run the service manually with

cdc_stream start-service

and pass the flags as command line arguments. When you run the service manually, the flag --log-to-stdout is particularly useful as it logs to the console instead of to the file.

cdc_rsync always logs to the console. To increase log verbosity, pass -vvv for debug logs or -vvvv for verbose logs.

For both sync and stream, the debug logs contain all SSH and SFTP commands that are attempted to run, which is very useful for troubleshooting. If a command fails unexpectedly, copy it and run it in isolation. Pass -vv or -vvv for additional debug output.