What is maximum SMB2 IOCTL(FSCTL_GET_SHADOW_COPY) response size.

Richard Sharpe realrichardsharpe at gmail.com
Mon Jan 19 15:25:33 MST 2015

On Mon, Jan 19, 2015 at 2:22 PM, Hemanth Thummala
<hemanth.thummala at gmail.com> wrote:
> Thanks all for your responses.
> As Stefan(metze) and Richard pointed out, this looks like SMB2.0 limitation
> to me. In negotiate process, max trans size is limited to 64K if we are not
> supporting large MTU or multi credits.
> I have tried modifying this max trans size from 64KB to 256KB. Could see
> that it negotiating 256K with client. But there is no difference in client
> behavior when trying to send huge response(~90K). Not sure if this is SMB2.0
> client's limitation. Or we might need to port the multi-credits and large
> MTU support to really support huge response buffer size.
> If there is a clear limitation for smb2.0 protocol, we might probably need
> to truncate the response to 64K.

There is a strict 64kiB transfer size limit for 2.02 which includes
IOCTL responses, I believe.

That is why you will need to either move to 4.0 or back-port the 4.0
stuff, both of which are going to be hard, or limit your responses to
64kiB (which will mean reducing the number of snapshots you report on
as well.)

Richard Sharpe

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