Transfer Resume

WinSCP supports resuming file transfers with SFTP protocol only. SCP protocol does not allow it at all.

For performance reasons resuming file transfer is supported only for files larger then a given threshold. By default the threshold is 100 kB. This can be configured on Resume tab of Preferences dialog. Also resuming text mode transfers is not supported.

When transferring file, it is first transferred into a temporary file with .filepart extension. Only after the transfer is completed the extension is removed.

If the transfer is interrupted, by user or because of dropped connection, partial file (with .filepart extensions) remains in the target folder. Once you attempt to transfer the same file again, WinSCP looks for the partial file. If it finds any, it offers you chance to resume the transfer (the confirmation can be disabled).

Note that partial files can be distinguished in file panel by special “torn sheet” overlay icon.

The above method of resuming file transfers is supported only for SFTP version 2 and newer (most of the current SFTP servers). There is another method supported even for the older servers (but not for SFTP version 4 and newer for ASCII/text transfers). Even if your server supports SFTP version 2 and never, you may find the method useful. Note that you must have overwrite confirmations enabled to use the method.

When you attempt to transfer file that already exists in target directory, the overwrite confirmation dialog is displayed. It offers Append button. By default it appends source file to the end of the target file. But if the target file is smaller then the source file, another prompt will be displayed offering you either to append source file or to resume transfer. If you select to resume transfer, the part of source file equal to size of the target file will be skipped and only the remaining part will be transferred.

The method is particularly useful for resuming transfers initiated by other clients and also to update log files.

It is on user responsibility that he/she resumes transfer with the same version of source file as the transfer was interrupted. Otherwise you will end with target file consisting partly of previous version and partly of current version of the source file. If the partial file is bigger than the source file, WinSCP will display error and disallow resume.

If the target file already exists and it is not write-able, but the target directory is, the file will may be transferred anyway. The error overwriting the file will occur only after the transfer finishes. It is because the file is first transferred into temporary file.

With SFTP protocol, when connection is broken, WinSCP can automatically reconnect and resume the transfer1. This can be configured on Resume tab of Preferences dialog.