Re: [PATCH 3/3] iscsi_tcp: Enable large commands

[Benny Halevy <bhalevy.lists@gmail.com>]

On Apr. 15, 2008, 16:38 +0300, Boaz Harrosh <bharrosh@panasas.com> wrote:
> On Tue, Apr 15 2008 at 16:08 +0300, James Smart <James.Smart@Emulex.Com> wrote:
>> Boaz Harrosh wrote:
>>> Let through up to the largest command of 260 defined by the scsi standard.
>>> iscsi core supports this already. Now that the scsi-ml supports it we can
>>> start using large commands.
>> Just to double-check:
>> if we use sg_io to pass through requests from user-land, which are addressed to
>> drivers or transports, and which may not be a SCSI command but rather just something
>> utilizing the sg_io infrastructure for management or pass-thru, are we limited to
>> 260 bytes for request/cmd payload ?
>>
>> -- james s
> 
> I think that only bsg, after the pending patches get accepted, will let you
> pass larger then 16 bytes commands from user-land. sg_io is I think limited 
> to 16. However this could be fixed. (With sg_io, did you mean commands going
> through the sg.c driver?)
> 
> The block layer imposes no limits, and so is scsi-ml. The only limits are from
> ULDs, and LLDs, like here, iscsi protocol does not support more then 
> SCSI_MAX_VARLEN_CDB_SIZE (260).

Actually the 260 byte limit comes from SCSI variable length CDB maximum length.
iSCSI allows to send larger CDBs with the Extended CDB AHS that has a 16 bit
length field. Within the protocol constraints, the longest CDB would be
16 (in BHS) + 65532 (in AHS) = 65548 bytes long :)

Benny

> 
> If you need support in any other ULD, I will be happy to help.
> 
> Boaz
> 
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