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smb v/very slow - help setting analysis steps in priority order

asked 2022-08-01 17:32:00 +0000

ajaznawaz gravatar image

updated 2022-08-04 09:05:20 +0000

grahamb gravatar image

Currently 500mb file taking over 2hrs to transfer over 100mb/s MPLS WAN link.

I can set captures client and server side, but in terms of analysis how can I priorities the steps..?

iPerf sprung to mind just to sanity check BW, latency is consistently 50-60ms which seem reasonable (Chennai IND <> SG)

I have ordered the packets in respect to delta-time, and another thing I noticed was a very signification number of dup-acks, probably few thousand in this particular pcap file.

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answered 2022-08-02 22:30:20 +0000

André gravatar image

SMB was not designed to be used over a WAN.
For example if you are using SMB version 1 or 2 the maximum block size is 64 kb. With a Round Trip Time (RTT) of 60 msec the maximum throughput will be 8,74 Mbps, regardless of the higher available bandwidth.

But when a 500 MiB file takes 2 hours then the average speed is 622 kbps. So bandwidth is not the issue (unless the WAN link is saturated with other traffic).

I noticed was a very signification number of dup-acks, probably few thousand in this particular pcap file.

A few thousands is still be less than 1 percent packets on a 500 MiB file. Anyway, check if TCP SACK is enabled (and used) and Window scaling is enabled/used in case of SMBv3.

But it is better to switch to a protocol that performs well over a WAN. Like (S)FTP, FTPS, HTTP(S).

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Hi Andre - thanks for replying, actually very interesting to know.

I checked few things:

  • SMBv2 is running
  • Majority of packets segment length circa 250bytes

I will look into your suggestion for using SMBv3. I also found this as the hosting environment is MS Azure

You made me feel a bit better now - thank you so much Andre !

ajaznawaz gravatar imageajaznawaz ( 2022-08-03 07:49:40 +0000 )edit

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Asked: 2022-08-01 17:32:00 +0000

Seen: 89 times

Last updated: Aug 04