Re: CAM

Andrew Valencia <vandys@cisco.com> writes:
> CAM - Adaptor-specific management
  ^^^
  SIM
> XPT - "Transport" layer. Apparently routes between PDRV and a CAM
> PDRV - Disk/tape/etc. code for creating SCSI commands
>
> My hope is that XPT can be boiled down to choosing where to connect
> from PDRV to CAM, and thus doesn't need to be a distinct process.
               ^^^
               SIM
> I don't understand its functionality well enough to comment on if
> this would work... Mike?

Please replace "CAM" with "SIM" in most places in your article.
The SIM's (SCSI Interface Modules) are responsible for adaptor-specific
management.

I think that while the XPT should be a distinct process, it doesn't
need to be in the path of most I/O transactions. The reason that the
XPT should be a distinct process is that it owns several tables,
including a table of SIM's currently running and a device table. Also,
the XPT layer interfaces to both the PDRV's (eg, send a CCB to the
appropriate SIM) and the SIM's (eg, tell the XPT I'm here).

As for SCSI requests, each PDRV sends a CCB to the appropriate SIM via
a function called xpt_action(). Now, xpt_action() could send the CCB to
the XPT process, and let the XPT process send the CCB to the
corresponding SIM. On the other hand, xpt_action() could contain a read
only copy of the XPT processes tables and route the request directly,
bypassing the XPT process. On the way back, the completion "callback"
would normally bypass the XPT layer anyway.

> My filesystem will definitely be able to do contiguous 64K reads, so you're
> looking at amortizing 64K of I/O over one of these turnarounds. ...

Is this data copied, or, as Jonathon (jont@hsa.com.au) suggested
previously, is the associated buffer shared among the servers in the
I/O path?

                                        Mike Larson
Received on Wed Sep 15 19:42:10 1993