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I have found that downloading a file in WinSCP via a symlink to the file, as opposed to the file itself, reduces the transfer speed by a factor of ~8. I'll list the various conditions I've tested this on at the end, suffice to say it seems quite persistent. I'm not able to find reference to this issue in the bug tracker, nor in any of the similar posts I've reviewed here on SU. Is this a new bug?

Conditions

  • Observed this pattern for several months
  • Have tried transferring from and to (at least) two different computers for most tests
  • That means 2 "from" computers and 2 "to" computers
  • Always on a Windows machine downloading from a Linux machine
  • All computers on different networks
  • All connections are wired
  • All transfer speeds are <25% of connection capacity, usually <10%
  • Using large files
  • Latest v5.15.1 (build 9407) WinSCP
  • Default WinSCP (SFTP) transfer options

Tests

  • Transfer of individual file vs. symlinked file
  • Transfer of folder with files vs. folder with symlinked files
  • Performance tests run simultaneously on the same systems and files to minimize extraneous variables
  • Verified psftp, PuTTY's SFTP upon which WinSCP is built, does not exhibit issue
  • Verified sftp command in Cygwin, does not exhibit issue
  • Verified scp-based connection in WinSCP does not exhibit issue
  • Verified transfer speeds are stable (i.e. self-consistent) across tests for a given set of conditions
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The current version of WinSCP (5.16) is not considering a target file size when employing download optimization strategies. Hence the low transfer rate.

The next version will do: https://winscp.net/tracker/1747


Btw, WinSCP is not using psftp SFTP code. Only PuTTY SSH code.

| improve this answer | |
  • So this slowdown is because WinSCP is grabbing overly small packets or something? Thanks for the SFTP/SSH comment, note that WinSCP's "slow FAQ" mentions PSFTP, which is why I presumed that was the the correct point of comparison. – TTT May 24 '19 at 17:45
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    Yes, small packets and short packet window/queue. – Martin Prikryl May 24 '19 at 18:08

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