SCSI target subsystem

Vladislav Bolkhovitin
2008-05-02T18:17:22Z

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James Bottomley wrote:
> On Fri, 2008-05-02 at 18:06 +0200, Bart Van Assche wrote:
>> James Bottomley clearly expressed in that thread that he doesn't want
>> to maintain two SCSI target frameworks. So what I propose is that SCST
>> is included in the mainline and afterwards that it is evaluated
>> whether or not it is desirable to keep other target code in the
>> mainline kernel.
>
> That's hardly sufficient. STGT is already in use. Their either has to
> be a migration path or, the preferred option, take the pieces of SCST
> that are actual improvements and embed them in STGT.

Actually, between SCSI initiator and target subsystems there is almost
*nothing* in common. This claim, at first glance, looks pretty wrong,
because both serve SCSI, so they must have a lot common. But look deeper
and you quickly find out that the majority of functionality as well as
data they use are dedicated for each subsystem, not shared.

Just look at SCST/qla2x00t/(changes done in the initiator qla2xxx driver
to support target mode, patch attached): 90% of changes is adding
callbacks for external target add-on, the rest is support for older,
than 2.6.17, kernels and sysfs magic. Note, no data are common between
initiator and target parts in the meaning that they both use them.

Then look at SCST (http://scst.sf.net). It implements complete
pass-through SCSI support and look how it interacts with initiator SCSI
subsystem. It calls only 2 functions: FIFO version of
scsi_execute_async() (original scsi_execute_async() provides
unacceptable LIFO commands order) and scsi_reset_provider() for task
management. And there is only one common variable: struct scsi_device.
That's all! In other storage modes (FILEIO/BLOCKIO) there is nothing
common with SCSI initiator subsystem at all.

Finally, try to find out in SCST any duplicated functionality.

Now, let's look how SCSI target/initiator integration currently done in
the kernel. For me it looks pretty artificial. For example, if I make a
general purpose kernel, for which 1% of users would run target mode, I
would have to enable as module "SCSI target support" as well as SCSI
target support for transport attributes. Now 99% of users of my kernel,
who don't need SCSI target, but need SCSI initiator drivers, would have
to have scsi_tgt loaded, because transport attribute drivers would
depend on it:

# lsmod
Module Size Used by
qla2xxx 130844 0
firmware_class 8064 1 qla2xxx
scsi_transport_fc 40900 1 qla2xxx
scsi_tgt 12196 1 scsi_transport_fc
brd 6924 0
xfs 511280 1
dm_mirror 24368 0
dm_mod 51148 1 dm_mirror
uhci_hcd 21400 0
sg 31784 0
e1000 114536 0
pcspkr 3328 0

No target functionality needed, but target mode subsystem is needed. Is
it a good design?

I wrote all above to support my at first glance shocking conclusion that
SCSI target subsystem is completely new subsystem of the kernel and it
should live on its own with its own maintainer! This is the same as with
the current interaction between SCSI and block subsystems in the kernel:
SCSI uses block's functionality, but that doesn't mean that block and
SCSI are the same subsystem.

Thus, how IMHO initiator and target drivers should be written:

- All initiator drivers should live in the SCSI initiator subsystem
(aka current SCSI subsystem) only, the same as today.

- All target drivers should live in the SCSI target subsystem only and
be either add-ons to initiator drivers, like, e.g., qla2x00t, or be a
complete driver, like, e.g., iSCSI-SCST.

Vlad

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