Parallel transfers with sftp (call for testing / advice)

Matthieu Hautreux matthieu.hautreux at
Tue May 5 08:41:09 AEST 2020

Le 10/04/2020 à 01:55, Darren Tucker a écrit :
> On Thu, 9 Apr 2020 at 01:34, Cyril Servant <cyril.servant at> wrote:
> [...]
>> Each of our front
>> nodes has an outgoing bandwidth limit (let's say 1Gb/s each, generally more
>> limited by the CPU than by the network bandwidth),
> You might also want to experiment with the Ciphers and MACs since
> these can make a significant difference in CPU utilization and, if
> that's the bottleneck, your throughput.  Which one is best will vary
> depending on your hardware, but it's likely to be either AES GCM if
> the hardware has AES instructions or chacha20-poly1305 if not.
> In the first example below the bottleneck is the source's relatively
> elderly 2.66GHz Intel CPU.  In the second it's the gigabit network
> between them.
> $ scp -c aes256-ctr -o macs=hmac-sha2-512
> ubuntu-18.10-desktop-amd64.iso.bz2 nuc:/tmp/
> ubuntu-18.10-desktop-amd64.iso.bz2            100% 1899MB  63.5MB/s   00:29
> $ scp -c chacha20-poly1305 at
> ubuntu-18.10-desktop-amd64.iso.bz2 nuc:/tmp/
> ubuntu-18.10-desktop-amd64.iso.bz2            100% 1899MB 112.1MB/s   00:16

As Cyril said, we are aware of the cpubound aspect of the available 
ciphers and MACs in OpenSSH and have already selected the most efficient 
one for our transfers after several benchmarking sessions.

Current hardware processors have a limited core capacity. Core 
frequencies are staying roughly at the same level since many years now 
and only core count are increasing, relying on developpers to play with 
parallelism in order to increase the compute throughput. The future does 
not seem brighter in that area.

In the meantime, network bandwidth has still increased at a regular 
pace. As a result, a cpu frequency that was once sufficient to fill the 
network pipe is now only at a fraction of what the network can really 
deliver. 10GE ethernet cards are common nowadays on datacenter servers 
and no openssh ciphers and MACs can deliver the available bandwidth for 
single transfers.

Introducing parallelism is thus necessary to leverage what the network 
hardware can offer.

The change proposed by Cyril in sftp is a very pragmatic approach to 
deal with parallelism at the file transfer level. It leverages the 
already existing sftp protocol and its capability to write/read file 
content at specified offsets. This enables to speed up sftp transfers 
significantly by parallelizing the SSH channels used for large 
transfers. This improvement is performed only by modifying the sftp 
client, which is a very small modification compared to the openssh 
codebase. The modification is not too complicated to review and validate 
(I did it) and does not change the default behavior of the cli.

It exists tools that offers parallel transfers of large files but we do 
really want to use OpenSSH for that purpose because it is the only 
application that we can really trust (by the way, thank you for making 
that possible). I do no think that we are the only one to think like 
this and I am pretty sure that such a change in the main code base of 
OpenSSH would really help users to use their hardware more efficiently 
in various situations.

Best regards,


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