What is maximum SMB2 IOCTL(FSCTL_GET_SHADOW_COPY) response size.

Jeremy Allison jra at samba.org
Mon Jan 19 00:27:04 MST 2015

On Sun, Jan 18, 2015 at 09:55:47PM -0800, Hemanth Thummala wrote:
> Hi,
> We are using samba 3.6.12+ stack. We are implemented windows previous
> version(shadow copies) support with new VFS module.
> On one of our nodes, there are huge number(~2K) of snapshots and we trying
> to send them(as list) as part of IOCTL response for FSCTL_GET_SHADOW_COPY.
> Response size is coming around 90K. Client is facing an issue in receiving
> the huge response packet. After receiving around 65K data( calculated
> acknowledged TCP segments), client returning TCP zero size recv window
> error followed by connection reset. Client behavior is same irrespective of
> version. I have tested on win 7, windows 8 and win2K12 server.
> Would like to know if there is any limit on this IOCTL response. Client
> negotiated 64K as max transaction size but this is the request buffer size
> that server can accept. Did not find any documentation about SMB2 client
> limits.
> We couldn't verify this against windows as windows supports only 512 VSS
> copies in back end. So it will never need to send more than 512 snapshots
> to client.

Sounds like you're hitting the trans2 limit on SMB1.

That limit is 1MB on SMB2 so it should work there.

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