Date: Thu, 29 Jan 2004 13:15:09 +1100 (EST) From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: FreeBSD-gnats-submit@FreeBSD.org Subject: bin/62055: scp(1) incorrectly reports "stalled" on slow copies Message-ID: <200401290215.i0T2F9Ec053406@gsmx07.alcatel.com.au> Resent-Message-ID: <200401290510.i0T5AEWC011070@freefall.freebsd.org>
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>Number: 62055 >Category: bin >Synopsis: scp(1) incorrectly reports "stalled" on slow copies >Confidential: no >Severity: non-critical >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Wed Jan 28 21:10:14 PST 2004 >Closed-Date: >Last-Modified: >Originator: Peter Jeremy >Release: FreeBSD 4.9-RELEASE-p1 i386 >Organization: Alcatel Australia Limited >Environment: System: FreeBSD gsmx07.alcatel.com.au 4.9-RELEASE-p1 FreeBSD 4.9-RELEASE-p1 #0: Fri Dec 19 07:22:50 EST 2003 root@aalp03.alcatel.com.au:/mnt/obj/mnt/rpc/src/sys/gsmx i386 >Description: By default, scp(1) will provide a progress meter showing the transfer ETA. If the link is slow, the transfer meter will alternate between displaying "- stalled -" and unrealistically short ETAs even though the actual connection is transferring data smoothly (as shown by tcpdump). By default, the progress meter is updated every second. If there has been no apparent progress in the transfer after 5 seconds, the progress meter will report "stalled" until some progress is reported. There appear to be two issues that will result in long delays between output progress being seen by the progress meter. Firstly, output from the scp process is in filesystem blocksize blocks - the number of bytes transferred (used by the progres meter) will only be incremented when a full block of data has been transferred. Therefore if the transfer rate is less than 1.6KB/sec (old 8K filesystem) or 3.2KB/sec (newer 16KB filesystem) then the link will report as "stalled". (Identified by code inspection). Secondly, the ssh process spawned by the scp process to perform the actual encryption and transfer includes a substantial internal buffer (>64KB) and appears to implement hysteresis. ktrace output of a sample transfer shows a peak of over 96KB buffered - at which point the ssh process stops reading until the buffer drops to about 32KB. This implies that there is approximately 64KB hysteresis and a transfer rate below about 13KB/sec can result in "stalled" reports. This behaviour is undesirable and appears to be a recent change. >How-To-Repeat: Use ipfw/dummynet or similar to reduce the outgoing ssh bandwidth to a second system and transfer a file: # ipfw pipe 20 config queue 10 bw 80000 # ipfw add 1005 pipe 20 tcp from any to 192.168.164.18 22 # dd if=/dev/urandom of=data count=512 # scp data 192.168.164.18:/tmp >Fix: Unclear - the buffering in both scp and ssh as well as the hysteresis in ssh are beneficial to maximize transfer bandwidth and minimise context switching. It would not be desirable to reduce these sizes when ssh is used across a LAN. In the case of scp, changing from atomicio(write, ...) to write(...) would remove the requirement to write at least filesystem_blocksize bytes/sec to the remote system. In the case of ssh, the hysteresis needs to be adjusted based on the outgoing bandwidth - this could possibly be done by resetting the "don't read more" flag after (say) 1 second. >Release-Note: >Audit-Trail: >Unformatted:
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