Re: Remote SCSI Emulation

[David Lang]

the type of thing that Muthian is looking at is the ability to take a
Linux box, stuff a bunch of cheap IDE drives in it, use LVM, MD, etc in
linux to provide redundancy, snapshots, replication, etc and then drop in
a SCSI card and let another server then treat the linux box like a large
external disk.

David Lang


 On Wed, 3 Sep 2003, Richard B. Johnson wrote:

> Date: Wed, 3 Sep 2003 17:23:46 -0400 (EDT)
> From: Richard B. Johnson <root@xxxxxxxxxxxxxxxxxx>
> To: Muthian S <muthian_s@xxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: Remote SCSI Emulation
>
> On Wed, 3 Sep 2003, Muthian S wrote:
>
> > Hi,
> >
> > Certain SCSI adapters like the Adaptec AHA 29160 are reportedly capable of
> > acting as a target and can receive SCSI commands from initiators.  Such an
> > adapter can be used to facilitate remote SCSI emulation by a PC.
> > For instance, if two PCs have the adapter, the two adapters can be
> > directly connected by a SCSI bus and the second PC can in effect serve as
> > an "emulated SCSI disk".  Such a setup is extremely helpful in various
> > scenarios.
> >
> > However, for this to work, the OS on the second PC (which serves as the
> > emulated scsi disk) should be capable of handling incoming SCSI requests and
> > directing them to an appropriate software layer.  Apparently, the CAM
> > subsystem of FreeBSD has this capability.  I was wondering if there is a
> > similar mechanism in linux.
> >
> > It would be really helpful if people have comments on whether such a setup
> > is
> > possible in linux, and if yes, are there specific adapters that are known
> > to work in this fashion.
> >
> > thanks,
> > Muthian.
> >
>
> Many modern SCSI Adapters can receive SCSI commands. They are not,
> however, relayed to some "appropriate software layer". Instead, the
> driver will handle these commands and provide an appropriate
> abstraction layer to user-mode software. Anybody who wants,
> can make such a driver. Typically the SCSI 'device' becomes a
> "memory device" because this provides the largest possible communications
> capability (a memory device can be a DSP (or several), for instance).
>
> Analogic's AP-85, now obsolete was a SCSI "memory device". It
> could accumulate high-speed data then it could process it with
> code that was uploaded using the SCSI interface as well. It was
> quite a machine, now about 15 years out-of-date. The processing
> was done with 4 DSPs (TMS320C30) plus a 16-bit controller uP.
> This was designed long before anybody heard of SMP.
>
> These kinds of interfaces are quite out-of-date because of the
> relatively low speed at which they operate (75 Megabytes/second).
> Therefore, there is not much call for such interface drivers.
> Everybody wants at least 300 Megabytes/second now-days, preferably
> 4 times that. A typical high-speed interface to parallel DSP
> systems now-days will ... "do an infinite loop in a few hours..."
> -- that, from an also-obsolete Gray advertisement --
>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.4.22 on an i686 machine (794.73 BogoMips).
>             Note 96.31% of all statistics are fiction.
>
>
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