smb read/write block size

Christopher R. Hertel crh at
Tue Jan 17 04:14:29 GMT 2006


I remember some of the discussion you cite below.  There was also a related 
conversation regarding buffer sizes as used by the SMB_ECHO command.  The 
notes wound up here:

...not that any of that helps much with regard to the specific questions
you're asking.

I would take a look at a capture of the protocol negotiation phase and
see what numbers are being established at the start of the SMB session.

Hope that's at least a little bit useful.

Chris -)-----

Stuart Kendrick wrote:
> hi,
> i'm analyzing read/write performance to various CIFS servers.  the largest 
> read block size i'm seeing is 61,440 bytes and the largest write block 
> size is 32,768 bytes
> SMB Write AndX FID: 0x0009, 61440 bytes at offset 0
> ...
> SMB Read AndX Request, FID: 0x000b, 32768 bytes at offset 0
> what drives this choice?  is this something hard-wired into Microsofts 
> implementations of CIFS and the rest of us have just copied it?  does this 
> relate to how frequently CIFS wants to commit bytes to disk?  is there 
> some way to increase this (registry hacks under Windows, smb.conf fiddles 
> under Samba, etc.)?  [i'm not looking for specific registry hacks or 
> smb.conf lines here ... just a conceptual understanding of the magic 
> around '61440' and '32768'.]
> i've found a thread driven by hertel, with urban, minshall, and others 
> involved, which suggests that a 64K ceiling exists because 
> SMB_COM_WRITE/READ_ANDX is a USHORT, i.e. can only acknowledge 64K bytes 
> ... so since CIFS can't acknowledge more than 64K bytes, therefore CIFS 
> can't send more than 64K bytes.
> ===============================================================
> "Once again, that suggests that you could send a write of more than 64K. 
> The thing is, the SMB_COM_WRITE_ANDX response does *not* have an extension 
> field according to the docs:
> 	USHORT Count; Number of bytes written
> So the server can only report up to 64K written. There's no way to report 
> that more than 64K were written unless one of the Reserved fields is 
> actually the upper 16 bits of the Count."
> =============================================================
> would i be correct in pointing to the USHORT definition of 
> SMB_COM_WRITE_ANDX/SMB_COM_READ_ANDX as the proximate cause for a 64K 
> read/write limitation in CIFS?
> i can see room to improve CIFS performance significantly in the LAN 
> environment, and dramatically in the WAN environment (long, fat pipes), if 
> i can hack my clients and servers to increase their CIFS Read/Write block 
> size above 61440 ==> ergo my queries.
> --sk
> stuart kendrick
> fhcrc

"Implementing CIFS - the Common Internet FileSystem" ISBN: 013047116X
Samba Team --     -)-----   Christopher R. Hertel
jCIFS Team --   -)-----   ubiqx development, uninq.
ubiqx Team --     -)-----   crh at
OnLineBook --    -)-----   crh at

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