Re: Why SFTP performance sucks, and how to fix it

[Joseph Galbraith <galb-list%vandyke.com@localhost>]

> Do you have any way to fix the SSH level for SFTP uploads? An SSH
> client which knows it's being used for SFTP can change its own
> maximum window size, so that the server can send arbitrary
> quantities of data with no constraint beyond the TCP-level acks. But
> for uploading, you'd have to persuade the SSH _server_ to change its
> window size depending on the use the client planned to put the
> connection to. (Having the server unconditionally set all window
> sizes to INT_MAX is a silly idea, for multiplexing reasons.)
>  
Well, the problem is that subsystems take a channel of type a and turn
it into a channel of type b.  If the server knew up front what you wanted,
it could pick appropriate values.  I've always thought subsystems were
a bad idea.

That said, the server could notice when you make a sftp-subsystem request
and send you a window adjust message for window you haven't consumed.
(I don't believe anything in the drafts prohibits this.)  This would 
essentially
increase your total available window.

Unfortunately, there is nothing that can be done about the maximum
packet size on the channel (which also plays into this whole mess.)
Well, we could introduct a channel request to notify the remote pee
that the per channel max packet size has been changed could be sent.
If that channel request succeeds, then larger packets could be sent.

- Joseph