"Session" vs "Channel" (JSch and sshd)

Question

I'm looking to move small files (at unknown times and in unknown quantities) from one server to another using JSch - unfortunately, I tried to open up a session for each transfer which quickly bombed out (as MaxSessions was I guess 10?).

Then I opened one session (per host) with JSch, and simply created a channel for each action (a single send would create a channel, put the file, closed the channel), but I encountered the same thing (when trying to do it 10 or more at a time, I would get connections refused as some channels were still closing when another request came through).

So I've looked at sshd_config and figured out that they have a MaxSessions property which would come in quite useful here - but it got me wondering, is there not a MaxChannelsPerSession property, or am I getting confused with regard to what JSch calls a "channel" and what is an actual SSH channel.

I am using the most recent version of JSch as of 20120316.

(And most importantly, should I post this in a different place on the site? It's kind of programming related, but I guess is part sysadmin as well...)

Shanahan

Answer 1

I'm the author of JSch.

"MaxSessions" property on sshd_config will be recognized as "MaxChannelsPerSession" on OpenSSH's sshd.

Spriggs

A slight follow-up: is a session/channel thread safe? Could I create a session to be held globally (or a reference passed around) and then create channels with it in different threads without worrying, or should I be putting it all in a syncrhonized block? – Shanahan

@Spriggs can you please check out this SO: #23693143 – Greenaway

Answer 2

Just for another alternative:

For transfering files, JSch offers the ChannelSftp class. With it you can transfer multiple files one after another (or even in parallel) using only one channel (for each server).

I don't know if this offers any performance benefits, though.

Upper

Have you seen any examples of using a single ChannelSftp instance to transfer files in parallel ? I have been trying to dig one up. – Nadya

No, my knowledge of ChannelSftp is mostly theoretic in nature, by reading its code and the SFTP pre-RFC. – Mechanism

Off topic, but is there an equivalent for Apache Mina SSHD's client? I use this rather than JSCH and my app would benefit from a reusable channel. Currently I'm connecting and disconnecting for every single file which has a huge overhead. – Quenna

@Sridhar-Sarnobat I did never use that, but from a look at the documentation, I would say DefaultSftpClient represents a SFTP channel from the client side, and its read/write methods for transferring can be used multiple times. – Mechanism

Ah I see. I guess my existing code is already optimized then (though maybe I need to avoid calling stop or close at all levels (session, ssh client, sftp client). Thanks. – Quenna