Re: Integration of SCST in the mainstream Linux kernel

Nicholas A. Bellinger
2008-02-09T00:01:31Z

On Fri, 2008-02-08 at 17:42 +0300, Vladislav Bolkhovitin wrote:
> Nicholas A. Bellinger wrote:
> > On Thu, 2008-02-07 at 12:37 -0800, Luben Tuikov wrote:
> >
> >>Is there an open iSCSI Target implementation which does NOT
> >>issue commands to sub-target devices via the SCSI mid-layer, but
> >>bypasses it completely?
> >>
> >> Luben
> >>
> >
> >
> > Hi Luben,
> >
> > I am guessing you mean futher down the stack, which I don't know this to
> > be the case. Going futher up the layers is the design of v2.9 LIO-SE.
> > There is a diagram explaining the basic concepts from a 10,000 foot
> > level.
> >
> > http://linux-iscsi.org/builds/user/nab/storage-engine-concept.pdf
> >
> > Note that only traditional iSCSI target is currently implemented in v2.9
> > LIO-SE codebase in the list of target mode fabrics on left side of the
> > layout. The API between the protocol headers that does
> > encoding/decoding target mode storage packets is probably the least
> > mature area of the LIO stack (because it has always been iSCSI looking
> > towards iSER :). I don't know who has the most mature API between the
> > storage engine and target storage protocol for doing this between SCST
> > and STGT, I am guessing SCST because of the difference in age of the
> > projects. Could someone be so kind to fill me in on this..?
>
> SCST uses scsi_execute_async_fifo() function to submit commands to SCSI
> devices in the pass-through mode. This function is slightly modified
> version of scsi_execute_async(), which submits requests in FIFO order
> instead of LIFO as scsi_execute_async() does (so with
> scsi_execute_async() they are executed in the reverse order).
> Scsi_execute_async_fifo() added as a separate patch to the kernel.

The LIO-SE PSCSI Plugin also depends on scsi_execute_async() for builds
on >= 2.6.18. Note in the core LIO storage engine code (would be
iscsi_target_transport.c), there is no subsystem dependence logic. The
LIO-SE API is what allows the SE plugins to remain simple and small:

-rw-r--r-- 1 root root 35008 2008-02-02 03:25 iscsi_target_pscsi.c
-rw-r--r-- 1 root root 7537 2008-02-02 17:27 iscsi_target_pscsi.h
-rw-r--r-- 1 root root 18269 2008-02-04 02:23 iscsi_target_iblock.c
-rw-r--r-- 1 root root 6834 2008-02-04 02:25 iscsi_target_iblock.h
-rw-r--r-- 1 root root 30611 2008-02-02 03:25 iscsi_target_file.c
-rw-r--r-- 1 root root 7833 2008-02-02 17:27 iscsi_target_file.h
-rw-r--r-- 1 root root 35154 2008-02-02 04:01 iscsi_target_rd.c
-rw-r--r-- 1 root root 9900 2008-02-02 17:27 iscsi_target_rd.h

It also means that the core LIO-SE code does not have to change when the
subsystem APIs change. This has been important in the past for the
project, but for upstream code, probably would not make a huge
difference.

--nab