[CentOS] 3ware disk failure -> hang -- SCSI backplanes

[Bryan J. Smith]

Peter Arremann <loony at loonybin.org> wrote:
> Hmmm... how is it a different matter? A scsi backplane has
> very little logic onboard other than what is required for
the
> scsi id selection (unless that is hardwired too).

Depends on the backplane.  You should see what the Dell, IBM
and other servers have.  ;->

Furthermore, SCSI-2 is a very _rich_ protocol, including
being able to handle transient, loss, etc... of nodes.  Most
cards, when used with a backplane that supports such logic,
handles a lot.  The rich SCSI driver for the OS then
communicates much of that status back to the system.

People who expect a 3Ware, Areca or other card to provide
similar don't realize that these card's drivers are rather
"dumb" from the OS standpoint.  All of the "intelligence" is
on-board, in the firmware, driven by the on-board ASIC or
microcontroller.  That's why these features _only_ work when
the on-board ASIC or microcontroller is controlling the
array.

And not when it's just presenting the disk as a JBOD back to
the system, where it cannot manage it as such.

> I don't know any backplane that has logic that can
understand 
> the scsi protocol and could communicate drive removal or
> such to the controller... ?

Huh?  Read up on SCSI-2 with SCA.  Now if you're using some
cheap SCSI carriers, no, you're not going to get the same. 
But if you're using a standard server backplane -- especially
what you'd get in an IBM eServer or a Dell PowerEdge, then
yes, you're going to get various SCSI-2 control.

It's just a couple of key pins which the SCSI-2 host adapter
then handles, and then reports back via it's driver.

Again, 3Ware, Areca and other controllers don't have "broken"
drivers per se, they just are _not_ delivering the full
SCSI-2 capabilities that some other cards do.

Now it will be interesting to see if Serial Attached SCSI
(SAS) cards, which can use SATA drives, will still report
back SATA status via their SCSI-2 facilities.  I know some of
the SAF-TE standard is adopted in some SATA drives and
backplanes.


-- 
Bryan J. Smith     Professional, Technical Annoyance                      b.j.smith at ieee.org      http://thebs413.blogspot.com
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*** Speed doesn't kill, difference in speed does ***