Robert Jennings wrote:
* Vladislav Bolkhovitin (vst@xxxxxxxx) wrote:
Robert Jennings wrote:
What I meant that is that the kernel tgt code (scsi_tgt*) receives
SCSI commands from one lld and send them to another lld instead of
sending them to user space.
Although the approach of passing SCSI commands from a target LLD to an
initiator one without any significant interventions from the target
software looks to be nice and simple, you should realize how limited,
unsafe and illegal it is, since it badly violates SCSI specs.
I think that 'implemented cleanly' means that one scsi_host is assigned
to only one initiator.
Vladislav listed a number of issues that are inherent in an implementation
that does not have a 1:1 relationship of initiators to targets. The vscsi
architecture defines the 1:1 relationship; it's imposible to have more
than one initiator per target.
Just few small notes:
1. As I already wrote, complete 1:1 relationship isn't practically
possible, because there is always a local access on the target (i.e. one
more initiator) and you can't disable it on practice.
I was proposing a 1:1 relationship of initiator to target within the
target framework for in-kernel pass-through. We would still have the
case that local access on the target is possible; an administrator with
privileges neccessary to create a target would have the responsibility
to not then access the device locally.
This is no different than if I create my root file system on /dev/sda1,
I should not also 'dd' data to /dev/sda1 while the system is running.
It's a bad idea, but nothing stops me; however this is something that
only a root level user can do. This would be the same, these targets in
pass-through have permissions by default that do not allow local access
by non-root users.
In principle, yes, but, as usually, on practice it's not so easy. In
your file system example the device is accessed via the FS, which
provides a shared mode, and everybody doesn't have any need to do
anything directly with the device. But in case of non-disk devices they
are always accessed directly, so to explain your limitation you will
have to write it with HUGE letters everywhere. Once one SCST user
cleared Unit Attention on his exported tape device using st driver and
asked then me why it isn't delivered to his remote initiator.
2. 1:1 relationship is a serious limitation for usage cases like an SPI
tape library serving backup for several servers on an FC net.
Restricting the relationship to 1:1 would be for pass-through devices
only, this would not necessarily dictate other target types which could
be used for such cases.
The tape library from my example is the pass-through device. You can't
access a parallel SCSI (SPI) device on an Fibre Channel (FC) in any
other mode, right?
Vlad
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