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SFTP transfer slower than FTP

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Created by Pamela Pineda, last modified by Albert.Cerezo on Feb 27, 2019

Views: 2,536 Votes: 1 Revisions: 5

Updated February 27, 2019


SFTP is slower compared to FTP as the maximum size of the packets is dictated by the protocol itself. Each packet in SFTP is encrypted before being written to the outgoing socket from the client which is decrypted when received by the server. This, of course, leads to slow transfer rates but very secure transfer. Generally, the speed depends on the protocol, and the more security is applied the slower it is.


SFTP transfers are not streamed like FTP or HTTP transfers.  They're chunked with the client opening a remote file and requesting it in chunks (no larger than 32K at a time).  After each read request, the server replies with the chunk and the client issues another.  When you request a chunk beyond the end of the file, you either get a partial chunk or a response from the server indicating you've reached the end.  That's how a download is done.


Some SFTP client makes "x" requests at a time making the transfer faster while some SFTP client issues one request at a time. This is how they're attaining faster transfer speeds. Serv-U is constantly retrieving and sending data to the client reducing the amount of time that's spent with no data being sent.  The trick in this method is that chunks of data may arrive out of order.  So the client must perform writes to the file according to the window of the file that the original request corresponds to.


If you are looking for faster transfer speed, it is best to use FTPS which is also secured. Also, there is a case that slow transfer is because of Serv-U Logging. Please refer to the suggestions below to increase SFTP speed.


Serv-U MFT version 15.x.x


  • Save the logging to a file on a local drive.
  • Do not use (SSH detailed command, SSH detailed replies, HTTP commands, and HTTP replies)


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