Parallel transfers with sftp (call for testing / advice)

Matthieu Hautreux matthieu.hautreux at
Sat May 9 08:46:00 AEST 2020

Le 06/05/2020 à 03:16, Nico Kadel-Garcia a écrit :
> On Tue, May 5, 2020 at 4:31 AM Peter Stuge <peter at> wrote:
>> Matthieu Hautreux wrote:
>>> 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.
>> I think you make a compelling argument. I admit that I haven't
>> reviewed the patch, even though that is what matters the most.
>> I guess that noone really minds ways to make SFTP scale, but ever since
>> the patch was proposed I have been thinking that the paralell channel
>> approach is likely to introduce a whole load of not very clean error
>> conditions regarding reassembly, which need to be handled sensibly both
>> within the sftp client and on the interface to outside/calling processes.
>> Can you or Cyril say something about this?
> I find it an unnecessary feature given the possibilities of
> out-of-band parallelism with multiple scp sessions transmitting
> diferent manifests of files, of sftp to do the same thing, and of
> tools like rsync to do it more efficiently by avoiding replication of
> previously transmitted data and re-connection to complete partial
> transmisions. It sounds like a bad case of "here, let me do this at a
> different level of the stack" that is not normally necessary and has
> already been done more completely and efficiently by other tools.

I think you misunderstood the main point that is that we want to 
overcome the bandwidth limitation of a single SSH connection for 
transferring _very_large_ files.

A single SSH connection as a bandwidth limitation that is either the 
network bandwidth or the efficiency of the cipher/MAC on the less 
powerfull core of the two connected endpoints.

If you traditionnaly use 1GE network cards, you will probably not see 
that if you have a good processorand the right cipher/mac, as the 
network will be the bottleneck.

If you you are using 10GE (or more) network cards, you will see the cpu 
limitation, and will get to your bandwidth roofline at something very 
far from you network capacity.

You are right, a lot of things already exist to send properly a very 
large number of small files over multiple SSH connections and we are 
already using this kind of approaches for some use cases.

However, I am not aware of anything enabling to send _very_large_ files 
using mutiple SSH connections. The proposed patch do that.

Give it a try, and send or receive a single 5GB file using a 10GE 
network and you will better see the point. If you have a solution with 
current ssh/scp/sftp/rsync that enables to get the most of the network 
(>1GB/s), then surely the patches are useless. But I am pretty sure that 
you will experience a bandwidth about a few hundreds MB/s at most 
depending on the cores involved on both sides.

>> And another thought - if the proposed patch and/or method indeed will not
>> go anywhere, would it still be helpful for you if the sftp client would
>> only expose the file offset functionality? That way, the complexity of
>> reassembly and the associated error handling doesn't enter into OpenSSH.
> Re-assembly, eror handling, and delivery verification were done by
> rsync ages ago. It really seems like re-inventing the wheel.

In the proposed patch, no re-assembly is necessary outside of the sftp 
client, as the sftp protocol was sufficiently well designed to allow 
read/write from/to particular remote offsets in files.

I do not see the patch as reinventing the wheel, maybe more widening it 
to run on widen roads.



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