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SubjectRe: Blacklist for DPES-*


On Thu, 14 Jan 1999, Doug Ledford wrote:

> Thomas Bogendoerfer wrote:
> >
> > I found following in patch-2.2.0-pre7:
> >
> > +{"IBM","DPES-","*", BLIST_NOTQ | BLIST_NOLUN},
> >
> > Vendor: IBM Model: DPES-31080 !t Rev: S31K
> > Type: Direct-Access ANSI SCSI revision: 02
> > Detected scsi disk sda at scsi0, channel 0, id 0, lun 0
> > ncr53c810-0-<0,0>: tagged command queue depth set to 8
> >
> > I'm wondering why the disk above works with tagged command queuing since ages.
> > In the pre-7 patch it was only for the DPES-31080, rev S31Q, but now
> > it's for every DPES drive (or do I misunderstand something ?). Who thinks
> > it's a good idea to blacklist every DPES drive ?
>
> I did. I don't know about Gerard's driver, but mine specifically does print
> out whenever a device that we set up as tagged queueing capable rejects part
> of our tagged queueing operation. That's what prompted that change. As for
> the version issue, most drives in a single family respond identically. If
> Gerard can confirm that your drive was actually using tagged queueing and that
> the driver didn't silently disable it, then we can change this around.

The ncr53c8xx driver(s) never disable(s) permanently tagged command
queuing. It only sets the maximum numbers of tags to the number of
disconnected commands when a QUEUE FULL status is reported by the device
and dequeue all commands that are not started yet. It then increases this
max number of commands by 1 every 1000 good status received up to the
maximum configured, and so on ...

Obviously, if the device returns QUEUE FULL when only 1 or ZERO commands
are actually disconnected (happens on Atlas I with L912) tagged queueing
may then be temporarily disabled.

I donnot remember having received any problem report for TCQ involving a
DPES-31080. But since I donnot have such an hard disks, I cannot tell more
about these drives.

I have some drives that return QUEUE FULL sometimes:
Atlas I L912, Atlas II LXY4, Cheatah2 0004.
I didn't upgrade the Atlases since they are fine for testing and I never
lost a single bit using them. The Cheetah needs to be flooded with
something like 50 commands at a time with write cache enabled in order to
start returning QUEUE FULL. I didn't remember any of these disks having
ever rejected a SCSI command with TCQ enabled on my system.
BTW, could you let me know the way used by the drive to reject tagged
commands. Does it send a M_REJECT message when it is supplied with
the ORDER TYPE + TAG message?

AFAIR, I haven't any experience of serious TCQ failure with the
drives I have had to use. The IBM disks I have (S12 and DDRS) and some
others behaves well for me with TCQ.
May-be I am just lucky or donnot perform hard enough crash-tests on my
system.

In my opinion, a blacklist for TCQ that just disallows the feature is too
rude. It has been reported that some drives may work reasonnably using a
small number of tags, and even just using 2 tagged commands instead of
untagged commands may lower significantly command latency.
Replacing the NOTQ flag by some numbers of tags (maximum and suggested for
example) should probably be more relevant in my opinion. We should not
miss that for 2.3.

Regards,
Gerard.





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