What is maximum SMB2 IOCTL(FSCTL_GET_SHADOW_COPY) response size.

[Hemanth Thummala]

Hi Richard,

Here is the exchange.

1) Initially client asked for number of copies. Buffer size if 16 here.

SMB2 (Server Message Block Protocol version 2)
    SMB2 Header
    Ioctl Request (0x0b)
        StructureSize: 0x0039
        Function: FSCTL_GET_SHADOW_COPY_DATA (0x00144064)
        GUID handle File:
        Max Ioctl In Size: 0
        Max Ioctl Out Size: 16
        Flags: 0x00000001
            .... .... .... .... .... .... .... ...1 = Is FSCTL: True
        In Data: NO DATA
            Offset: 0x00000078
            Length: 0
        Out Data: NO DATA
            Offset: 0x00000078
            Length: 0

2) Server responded with 1730 volumes and informed client it requires ~86K
buffer to send the response.

SMB2 (Server Message Block Protocol version 2)
    SMB2 Header
    Ioctl Response (0x0b)
        StructureSize: 0x0031
        unknown: 0000
        Function: FSCTL_GET_SHADOW_COPY_DATA (0x00144064)
        GUID handle File:
        In Data: NO DATA
            Offset: 0x00000070
            Length: 0
        Out Data
            Offset: 0x00000070
            Length: 16
            Num Volumes: *1730*
            Num Labels: 0
            Count: *86506*
            Label:
            Label:
            Label: [ Null ]

3) Client requested back with 86K buffer to get the entire list.

SMB2 (Server Message Block Protocol version 2)
    SMB2 Header
    Ioctl Request (0x0b)
        StructureSize: 0x0039
        Function: FSCTL_GET_SHADOW_COPY_DATA (0x00144064)
        GUID handle File:
        Max Ioctl In Size: 0
        Max Ioctl Out Size: *86518*
        Flags: 0x00000001
            .... .... .... .... .... .... .... ...1 = Is FSCTL: True
        In Data: NO DATA
            Offset: 0x00000078
            Length: 0
        Out Data: NO DATA
            Offset: 0x00000078
            Length: 0

..
There were quite a few TCP segments carrying the snapshot details to client.
Client acknowledged few.
...
After a while client disconnected the session.
SMB2     138    [TCP ZeroWindow] [TCP ACKed unseen segment] Tree Disconnect
Request

Thanks,
Hemanth.

On Sun, Jan 18, 2015 at 10:01 PM, Richard Sharpe <
realrichardsharpe at gmail.com> wrote:

> On Sun, Jan 18, 2015 at 9:55 PM, Hemanth Thummala
> <hemanth.thummala at gmail.com> 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.
>
> What does the MaxOutputResponse field in the request show? That is a
> 32-bit field.
>
> --
> Regards,
> Richard Sharpe
> (何以解憂？唯有杜康。--曹操)
>