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The log file you sent me has no post-processing and it has completed successfully. Therefore we know that the actual backup does work.

What gives you the problem is transferring huge files over SFTP because it takes some time and that causes PHP to time out. Actually, we have documented why this happens. Please read the paragraphs below the image on that page.

So the answer to your question is that it's nobody's fault. The timeouts are there for a very important reason (to prevent buggy pages caught in an infinite loop or an unrealistically long calculation from bringing down the entire server). It's just how web servers work.

There are a few ways to go about it.

1. Recommended. Run your backup through the provided CLI script. PHP does not have a timeout limit in CLI mode. Moreover, you don't have Apache, with its own internal timeouts, in the way. Therefore any size part file, even a 2G one, can be transferred through SFTP without a problem. This is what I do for all my sites.

2. If you really need to run your backups through the web interface and use SFTP to upload the only solution is to use a smaller part size. Typically something in the range 10M to 50M works but you need to experiment since the value depends on the speed of both servers (web server and the SFTP target). Start high (100M) and keep decreasing the part size until you find the sweet spot.

3. Use a remote storage which allows chunked uploading such as Amazon S3, Google Storage, BackBlaze, Dropbox, Google Drive or OneDrive. All of them allow you to have a large part size e.g. 2G but they can transfer your backup archives in smaller chunks of 5M to 20M. Since the work item is a chunk we can transfer just as many chunks as possible without triggering a timing out, then continue the upload in the next page load, repeat until the entire archive is transferred. This method is the best for transferring large files but requires the remote storage to support the reconstruction of a big file out of smaller chunks. SFTP does not have such a feature, that's why it's not possible to do that with SFTP.