pdiffcopy 1.0.1

Description:

pdiffcopy 1.0.1

The pdiffcopy program synchronizes large binary data files between Linux
servers at blazing speeds by performing delta transfers and spreading its work
over many CPU cores. It’s currently tested on Python 2.7, 3.5+ and PyPy (2.7)
on Ubuntu Linux but is expected to work on most Linux systems.


Status
Installation
Command line
Benchmarks

Low concurrency
High concurrency
Silly concurrency


Limitations
History
About the name
Contact
License



Status
Although the first prototype of pdiffcopy was developed back in June 2019 it
wasn’t until March 2020 that the first release was published as an open source
project.

Note
This is an alpha release, meaning it’s not considered mature and
you may encounter bugs. As such, if you’re going to use pdiffcopy,
I would suggest you to keep backups, be cautious and sanity check
your results.

There are lots of features and improvements I’d love to add but more
importantly the project needs to actually be used for a while before
I’ll consider changing the alpha label to beta or mature.


Installation
The pdiffcopy package is available on PyPI which means installation should be
as simple as:
$ pip install 'pdiffcopy[client,server]'
There’s actually a multitude of ways to install Python packages (e.g. the
per user site-packages directory, virtual environments or just installing
system wide) and I have no intention of getting into that discussion here, so
if this intimidates you then read up on your options before returning to these
instructions 😉.
The names between the square brackets (client and server) are called
“extras” and they enable you to choose whether to install the client
dependencies, server dependencies or both.


Command line
Usage: pdiffcopy [OPTIONS] [SOURCE, TARGET]
Synchronize large binary data files between Linux servers at blazing speeds
by performing delta transfers and spreading the work over many CPU cores.
One of the SOURCE and TARGET arguments is expected to be the pathname of a
local file and the other argument is expected to be a URL that provides the
location of a remote pdiffcopy server and a remote filename. File data will be
read from SOURCE and written to TARGET.
If no positional arguments are given the server is started.
Supported options:






Option
Description



-b, --block-size=BYTES
Customize the block size of the delta transfer. Can be a plain
integer number (bytes) or an expression like 5K, 1MiB, etc.

-m, --hash-method=NAME
Customize the hash method of the delta transfer (defaults to ‘sha1’
but supports all hash methods provided by the Python hashlib module).

-W, --whole-file
Disable the delta transfer algorithm (skips computing
of hashing and downloads all blocks unconditionally).

-c, --concurrency=COUNT
Change the number of parallel block hash / copy operations.

-n, --dry-run
Scan for differences between the source and target file and report the
similarity index, but don’t write any changed blocks to the target.

-B, --benchmark=COUNT
Evaluate the effectiveness of delta transfer by mutating the TARGET
file (which must be a local file) and resynchronizing its contents.
This process is repeated COUNT times, with varying similarity.
At the end an overview is printed.

-l, --listen=ADDRESS
Listen on the specified IP:PORT or PORT.

-v, --verbose
Increase logging verbosity (can be repeated).

-q, --quiet
Decrease logging verbosity (can be repeated).

-h, --help
Show this message and exit.





Benchmarks
The command line interface provides a simple way to evaluate the effectiveness
of the delta transfer implementation and compare it against rsync. The tables
in the following sections are based on that benchmark.


Low concurrency
High concurrency
Silly concurrency



Low concurrency

Concurrency:
6 processes on 4 CPU cores

Disks:
Magnetic storage (slow)

Filesize:
1.79 GiB


The following table shows the results of the benchmark on a 1.79 GiB
datafile that’s synchronized between two bare metal servers that each
have four CPU cores and spinning disks, where pdiffcopy was run with
a concurrency of six [1]:


Delta
Data size
pdiffcopy
rsync



10%
183 MiB
3.20 seconds
38.55 seconds

20%
366 MiB
4.15 seconds
44.33 seconds

30%
549 MiB
5.17 seconds
49.63 seconds

40%
732 MiB
6.09 seconds
53.74 seconds

50%
916 MiB
6.99 seconds
57.49 seconds

60%
1.07 GiB
8.06 seconds
1 minute and 0.97 seconds

70%
1.25 GiB
9.06 seconds
1 minute and 2.38 seconds

80%
1.43 GiB
10.12 seconds
1 minute and 4.20 seconds

90%
1.61 GiB
10.89 seconds
1 minute and 3.80 seconds

100%
1.79 GiB
12.05 seconds
1 minute and 4.14 seconds





[1]
Allocating more processes than there are CPU cores available can make
sense when the majority of the time spent by those processes is waiting
for I/O (this definitely applies to pdiffcopy).




High concurrency

Concurrency:
10 processes on 48 CPU cores

Disks:
NVMe (fast)

Filesize:
5.5 GiB


Here’s a benchmark based on a 5.5 GB datafile that’s synchronized between two
bare metal servers that each have 48 CPU cores and high-end NVMe disks, where
pdiffcopy was run with a concurrency of ten:


Delta
Data size
pdiffcopy
rsync



10%
562 MiB
4.23 seconds
49.96 seconds

20%
1.10 GiB
6.76 seconds
1 minute and 2.38 seconds

30%
1.65 GiB
9.43 seconds
1 minute and 13.73 seconds

40%
2.20 GiB
12.41 seconds
1 minute and 19.67 seconds

50%
2.75 GiB
14.54 seconds
1 minute and 25.86 seconds

60%
3.29 GiB
17.21 seconds
1 minute and 26.97 seconds

70%
3.84 GiB
19.79 seconds
1 minute and 27.46 seconds

80%
4.39 GiB
23.10 seconds
1 minute and 26.15 seconds

90%
4.94 GiB
25.19 seconds
1 minute and 21.96 seconds

100%
5.43 GiB
27.82 seconds
1 minute and 19.17 seconds



This benchmark shows how well pdiffcopy can scale up its performance by running
on a large number of CPU cores. Notice how the smaller the delta is, the bigger
the edge is that pdiffcopy has over rsync? This is because pdiffcopy computes
the differences between the local and remote file using many CPU cores at the
same time. This operation requires only reading, and that parallelizes
surprisingly well on modern NVMe disks.


Silly concurrency

Concurrency:
20 processes on 48 CPU cores

Disks:
NVMe (fast)

Filesize:
5.5 GiB


In case you looked at the high concurrency benchmark above, noticed the large
number of CPU cores available and wondered whether increasing the concurrency
further would make a difference, this section is for you 😉. Having taken the
effort of developing pdiffcopy and enabling it to run on many CPU cores I was
curious myself so I reran the high concurrency benchmark using 20 processes
instead of 10. Here are the results:


Delta
Data size
pdiffcopy
rsync



10%
562 MiB
3.80 seconds
49.71 seconds

20%
1.10 GiB
6.25 seconds
1 minute and 3.37 seconds

30%
1.65 GiB
8.90 seconds
1 minute and 12.40 seconds

40%
2.20 GiB
11.44 seconds
1 minute and 19.57 seconds

50%
2.75 GiB
14.21 seconds
1 minute and 25.43 seconds

60%
3.29 GiB
16.45 seconds
1 minute and 28.12 seconds

70%
3.84 GiB
19.05 seconds
1 minute and 28.34 seconds

80%
4.39 GiB
21.95 seconds
1 minute and 25.49 seconds

90%
4.94 GiB
24.60 seconds
1 minute and 22.27 seconds

100%
5.43 GiB
26.42 seconds
1 minute and 18.73 seconds



As you can see increasing the concurrency from 10 to 20 does make the benchmark
a bit faster, however the margin is so small that it’s hardly worth bothering.
I interpret this to mean that the NVMe disks on these servers can be more or
less saturated using 8–12 writer processes.

Note
In the end the question is how many CPU cores it takes to saturate
your storage infrastructure. This can be determined through
experimentation, which the benchmark can assist with. There are no
fundamental reasons why 30 or even 50 processes couldn’t work well,
as long as your storage infrastructure can keep up…




Limitations
While inspired by rsync the goal definitely isn’t feature parity with rsync.
Right now only single files can be transferred and only the file data is
copied, not the metadata. It’s a proof of concept that works but is limited.
While I’m tempted to add support for synchronization of directory trees and
file metadata just because its convenient, it’s definitely not my intention to
compete with rsync in the domain of synchronizing large directory trees,
because I would most likely fail.
Error handling is currently very limited and interrupting the program using
Control-C may get you stuck with an angry pool of multiprocessing workers that
refuse to shut down 😝. In all seriousness, hitting Control-C a couple of times
should break out of it, otherwise try Control-\ (that’s a backslash, it should
send a QUIT signal).


History
In June 2019 I found myself in a situation where I wanted to quickly
synchronize large binary datafiles (a small set of very large MySQL
*.ibd files totaling several hundred gigabytes) using the abundant
computing resources available to me (48 CPU cores, NVMe disks,
bonded network interfaces, you name it 😉).
I spent quite a bit of time experimenting with running many rsync processes in
parallel, but the small number of very large files was “clogging up the pipe”
so to speak, no matter what I did. This was how I realized that rsync was a
really poor fit, which was a disappointment for me because rsync has long been
one my go-to programs for ad hoc problem solving on Linux servers 🙂.
In any case I decided to prove to myself that the hardware available to me
could do much more than what rsync was getting me and after a weekend of
hacking on a prototype I had something that could outperform rsync even though
it was written in Python and used HTTP as a transport 😁. During this weekend
I decided that my prototype was worthy of being published as an open source
project, however it wasn’t until months later that I actually found the time to
do so.


About the name
The name pdiffcopy is intended as a (possibly somewhat obscure) abbreviation of
“Parallel Differential Copy”:

Parallel because it’s intended run on many CPU cores.
Differential because of the delta transfer mechanism.

But mostly I just needed a short, unique name like rsync so that searching for
this project will actually turn up this project instead of a dozen others 😇.


Contact
The latest version of pdiffcopy is available on PyPI and GitHub. The
documentation is hosted on Read the Docs and includes a changelog. For bug
reports please create an issue on GitHub. If you have questions, suggestions,
etc. feel free to send me an e-mail at [email protected].


License
This software is licensed under the MIT license.
© 2020 Peter Odding.