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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.
When transfering file, it is first transfered info temporary file with .filepart extension. Only after the transfer is completed the extension is removed.
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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).
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 transfered.
The method is particularly useful for resuming transfers initiated by other clients and also to update log files.
Common Problems
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 writeable, but the target directory is, the file will be transfered anyway. The error overwritting the file will occur only after the transfere finishes. It is because the file is first transfered into temporary file.