Re: remove exofs and the T10 OSD code V2

[Douglas Gilbert]

On 2018-11-01 1:03 a.m., Boaz Harrosh wrote:
> On 31/10/18 23:10, Douglas Gilbert wrote:
> > On 2018-10-31 4:57 p.m., Boaz Harrosh wrote:
> > > On 30/10/18 09:45, Christoph Hellwig wrote:
> > > > On Mon, Oct 29, 2018 at 02:42:12PM -0600, Jens Axboe wrote:
> > > > > LGTM, for both:
> > > >
> > > > I also have this one on top as requested by Martin.  The core block
> > > > bidi support is unfortunately also used by bsg-lib, although it is
> > > > not anywhere near as invasive.  But that is another argument for
> > > > looking into moving bsg-lib away from using block queues..
> > >
> > > BUT this patch is very very wrong.
> > >
> > > Totally apart from T10-OSD and its use in the field. Support for scsi BIDI
> > > commands is not exclusive to T10-OSD at all. Even the simple scsi-array
> > > command-set has BIDI operations defined. for example the write-return-xor
> > > and so on.
> > >
> > > Also some private administrative CDBs of some vendor devices uses SCSI-BIDI.
> > > So this patch just broke some drivers. (User-mode apps use bsg pass through)
> > >
> > > Also you might (try hard and) remove all usage of scsi-bidi as an initiator
> > > from the Linux Kernel. But what about target mode. As a target we have supported
> > > on the wire bidi protocols like write-return-xor and others for a long time.
> > > Are you willing to silently break all these setups in the field on the next update?
> > > Are you so sure these are never used?
> > >
> > > PLEASE, I beg of you guys. Do not remove SCSI-BIDI. It is a cry of generations.
> > >
> > > And I think by the rules of Linus, as far as target mode. You are not allowed
> > > to break users in this way.
> >
> > Hi,
> > I'm in the process of rebuilding the sg driver with the following goals:
> >
> >     - undo 20 years of road wear, some of which is caused by literally
> >       hundreds of "laparoscopic" patches that gradually ruin a driver,
> >       at least from the maintainer's viewpoint. Comments that once made
> >       sense become cryptic or just nonsense; naming schemes that
> >       obliterate variable names to the extent that a random name
> >       generator would be easier to follow; and just plain broken code.
> >       For example, why sort out the existing locking at a particular
> >       level when you can add a new lock in a completely non-orthogonal
> >       way? [Yes, I looking at you, google.] Anyway, my first cut at this
> >       is out there (on the linux-scsi list, see: "[PATCH v3 0/8] sg:
> >       major cleanup, remove max_queue limit"). Not much new there,
> >       unless you look very closely
> >
> >     - the next step is to add to the sg driver async SCSI command
> >       capability based on the sg_io_v4 structure previously only used
> >       by the bsg driver and now removed from bsg. The main advantage
> >       of the sg_io_v4 structure over previous pass-through interface
> >       attempts is the support of SCSI bidi commands
> >
> >     - as part of this effort introduce two new ioctls: SG_IOSUBMIT and
> >       SG_IORECEIVE to replace the write()/read() technique currently
> >       in use (since Linux 1.0 in 1992). The write()/read() technique
> >       seems to be responsible for various security folks losing clumps
> >       of their hair. One advantage of ioctls, as Alan Cox pointed out,
> >       is the ability to write to and read back from the kernel in a way
> >       that is naturally synchronized. [Actually, those security folks
> >       shouldn't look too closely at sg_read() in that respect.]
> >
> > In discussions with several folks who are on the T10 committee, I
> > wondered why there was no READ GATHERED command (there has been a
> > WRITE SCATTERED for 2 years). The answer was lack of interest ***,
> > plus the difficultly of implementation. You see, READ GATHERED needs
> > to send a scatter gather list to the device and get the corresponding
> > data back (as a linear array). And that requires either:
> >     a) bidi commands (scatter gather list in the data-out, corresponding
> >        "read" data in the data-in), or
> >     b) loooong SCSI commands, up to around 256 bytes long in which the
> >        sgat list is the latter part of that command
> >
> > And the T10 folks say neither of those options is well supported or
> > is expensive.
>
> It is supported in Linux scsi/osd driver is a proof of that. And expensive
> it is not. I have demonstrated the ability to saturate a 10G link over
> a raid of devices from a single writer. In OSD everything is bidi.
>
> > I'm guessing they are referring to Linux and Windows.
> > I haven't argued much beyond that point, but it looks like a bit of
> > a chicken and egg situation.
> >
> >
> > Don't know too much about the T10 OSD stuff. But I can see that it
> > uses both long SCSI commands and a lot of bidi. IMO it seems to be
> > 10 or 20 years before its time. Maybe ibm/redhat need to
> > (re-)discover it for it to catch on.
> >
> >
> > Plus there are proprietary SCSI bidi commands out there. People contact
> > me and ask me how to issue them with sg3_utils package. Easy, I tell them,
> > just use sg_raw with a bsg device. Typically, in my experience, "no news
> > is good news" after suggestions like that. When I give bad advice, I
> > usually hear back relatively quickly. Anyone who wants SCSI bidi _async_
> > support is currently out of luck.

And I forgot to mention that the scsi_debug driver has had an implementation
of the SCSI XDWRITEREAD(10) command with T10 semantics. That is a bidi
command and facilitates testing of SCSI bidi from other block/SCSI
drivers and user space programs. It has been there for over 4 years with
no problems reported. As mentioned above it can be exercised with the
sg_raw utility in the sg3_utils package via a bsg device.

Doug Gilbert