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HI, I 'am analysing very poor performances between a windows 7 pc and an Hitachi NAS Protocol used is SMB2. A sniffer trace has been made on the PC. Minimum bandwith between the two ends is 100 Mb/s :


-pc says MSS=1460, HNAS says MSS=1460

-pc says windows scaling factor 8 , HNAS says windows scaling factor 3

-RTT between the two ends =14ms

-No retansmission, no segment lost , no dup ack

-Troughput of transfer is about 1,2 Mb/s

The PC receive about 123 MB of applicative bytes(about 230 files) from the NAS and this takes about 970 s . (when the PC gets the same among of data (and same number of files) from a another remote site with an higher RTT of 23 ms , it takes only 600 s ..)

The PC always advertises a 64KB window (64 KB plateau curve). Bytes in flight (send by the NAS) show only a 2 KB plateau curve (since the PC requests only PDU of 2048 Bytes, I presume the server cannot sends more without an acknowledge, this may explain the seen value of bytes in flight)

My explanation for this slow transfer is :

The PC uses very often SMB2 READ REQUEST of 2048 Bytes instead of higher length (seen with wireshark io graph using "smb2.read_length" filter) This behavior generates too many applications turns and the latency makes the rest ...

Well, at those steps of my investigations my questions are :

How the heck does not the PC send SMB2 READ REQUESTs with a requested "read length" greater than 2048 ? Why is the PC advertised received windows at only 64KB (according it has a windows scaling factor of 8, it could increase at 16MB maximum..)

Any advice would be appreciated , regards jm

asked 27 Oct '15, 06:15

jmnogues's gravatar image

accept rate: 0%

Wireshark will tell you what is in the frames and you've found that yourself. However it won't be able to tell you why a system behaves in a certain way.

Why is the PC advertised received windows at only 64KB (according it has a windows scaling factor of 8, it could increase at 16MB maximum..)

sounds like a good question for the Microsoft support people.

BTW: Did you search this site for SMB? I believe we had similar cases, but I can't remember if there was a solution.

(27 Oct '15, 06:51) Kurt Knochner ♦

SMB2 has a 64k limit, can't remember the math. There is a registry setting.. don't quote me but I think it was DisableLargeMtu=0 in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\Tcpip\Parameters

Edit: it has to be enabled on both ends for it to work when both are windows boxes

(28 Oct '15, 00:47) DarrenWright

"SMB2 has a 64k limit, can't remember the math"

In the "SMB2 NEGOTIATE Response" of the server we see "MaxReadSize (4 bytes): The maximum size, in bytes, of the Length in an SMB2 READ Request that the server will accept. (

In my case the server is effectively giving this value 65536 which seems to be the default for SMB2.

So this would let think that when receiving a 64K Maxreadsize from the server, the client will unconditionaly sets its windows size to 64K.. why not , seems rather logical ..;-)

Anyway this is not the main issue and I would even be very happy if the 64K Windows was really used on the wire (i.e.if bytes in flight were 64K and not only 2K ..;-)
But as mentionned above , the client requests only 2048 bytes blocks despite the server can accept 65536 bytes blocks .. ;-(.


(28 Oct '15, 06:12) jmnogues
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question asked: 27 Oct '15, 06:15

question was seen: 2,863 times

last updated: 28 Oct '15, 06:12

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