Build the read-ahead mechanism into SftpFileStream #1705
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This change unifies the SFTP download implementations that exist via DownloadFile and via SftpFileStream, by rewriting SftpFileStream to perform the same "read-aheads" as DownloadFile. This brings the performance of downloads via SftpFileStream in line with DownloadFile, such that the latter is now effectively SftpFileStream.CopyTo. It also brings the recently added DownloadFileAsync up to speed since that was implemented via SftpFileStream.CopyToAsync. The methodology is a mix of the previous one and that within OpenSSH: the first call to SftpFileStream.Read sends one read request to the server. The second sends two and when not interrupted by Write or similar, the number of in-flight read requests continues to scale up in this fashion. I have measured CopyTo to be 3-20x faster than before, depending on file size and server round-trip time.
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Performance analysis: Summary: probable ~10% regression in DownloadFile for small files on high-latency connections. Otherwise, no statistically significant differences for DownloadFile. Tests were run for different file sizes against a geographically close server (Europe) and one far away (US West). 1MB, EuropeThe noisiest test. No statistically significant difference for DownloadFile.
1MB, West USThe DownloadFile difference is somewhat consistent
8MB, Europe
8MB, West US
64MB, Europe
64MB, West US
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WojciechNagorski
approved these changes
Oct 20, 2025
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This change unifies the SFTP download implementations that exist via DownloadFile and
via SftpFileStream, by rewriting SftpFileStream to perform the same "read-aheads" as
DownloadFile. This brings the performance of downloads via SftpFileStream in line with
DownloadFile, such that the latter is now effectively SftpFileStream.CopyTo. It also
brings the recently added DownloadFileAsync up to speed since that was implemented via
SftpFileStream.CopyToAsync.
The methodology is a mix of the previous one and that within OpenSSH: the first call to
SftpFileStream.Read sends one read request to the server. The second sends two and when
not interrupted by Write or similar, the number of in-flight read requests continues to
scale up in this fashion.
I have measured CopyTo to be 3-20x faster than before, depending on file size and server
round-trip time. See #1705 (comment)
closes #1644
contributes to #1331
fixes #1633
fixes #1618