smb read/write block size

Stuart Kendrick skendric at
Sun Jan 29 23:50:47 GMT 2006

hi chris,

ok, i've been chewing on this a while, analyzing Session Setup AndX 
Requests from various implementations ... reading your text at ... and i don't have a definitive 
answer (no surprise there).  a key test would be to watch two 64 bit 
versions of Windows copy files to one another ... but i haven't done that.

however here's what i believe ... at least for the moment:

-in Session Setup AndX Request, the initiator uses a USHORT to specify 
MaxBufferSize "Client's maximum buffer size" (SNIA 1.0) or 'Max Buffer' 
(Ethereal) ... ok, the use of a USHORT here places a 64K ceiling on 
read/write block sizes.

-in Read AndX Request, the initiator uses a USHORT to specify MaxCount 
"Max number of bytes to return" (SNIA 1.0) or 'Max Count Low' (Ethereal).  
and in Read AndX Response, the server uses a USHORT to specify Data Length 
"Number of data bytes" (SNIA 1.0) or 'Data Length Low' (Ethereal).  ... 
ok, the use of a USHORT places a 64K ceiling on read block size.

-in Write AndX Request, the initiator uses a USHORT to specify DataLength 
"Number of data bytes in buffer" (SNIA 1.0) or Data Length Low (Ethereal).  
and in Write AndX Response, the server uses a USHORT to specify Count 
"Number of bytes written" (SNIA 1.0) or 'Count Low' (Ethereal)  ... ok, 
the use of a USHORT here places a 64K ceiling on write block size.

summary ==> all these USHORTs suggest to me that 64K is the limit on 
read/write block size

caveat ==>

my read of Session Setup AndX Request says that if CAP_LARGE_FILE is set 
"The client can deal with files having 64 bit offsets" (SNIA v1.0) or 
'Large Files' (Ethereal), then Read AndX Request can employ a ULONG to 
specify MaxCountHigh "High 16 bits of MaxCount if CAP_LARGE_READX; else 
must be zero" (SNIA 1.0)

from p.70 of SNIA:
"If CAP_LARGE_READX [sp] was indicated by the server in the negotiate 
protocol response, the request's MaxCount field may exceed the negotiated 
buffer size if Fid refers to a disk file.  The server may arbitrarily 
elect to return fewer than MaxCount bytes in response."

i.e. seems to me that if the two sides agree to think in terms of 64 bit 
files, then they can sling TCP streams at each other which exceed 64K in 
length.  at least on the protocol level, i can see this.  on the 
implementation level ... i don't understand how i could figure out how 
much enough buffer space to set aside to handle the incoming flood.  maybe 
i would just query my TCP Window Size and allocate an SMB read buffer of 
the same size ... that feels gross

[i have not installed Samba or Windows on a 64 bit machine to see if 
either one sets CAP_LARGE_READX and then does what i describe above]

anyway, i'm going to go away believing that 32 bit clients can't bust the 
64K barrier on read/write block size ... and therefore must continue to 
toodle along with less-than-optimal file transfer performance, both on the 
LAN (SMB processing time for the Read AndX/Write AndX commands at 64K 
intervals) and on the WAN (can't have more than 64K in flight at a time).

if you think i'm mistaken, do let me know


stuart kendrick

On Mon, 16 Jan 2006, Christopher R. Hertel wrote:

> Stuart,
> 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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