Re: disk geometry (i386/amd64)

[Don NetBSD <netbsd-embedded%gmx.com@localhost>]

On 9/11/2018 11:16 AM, Mike Pumford wrote:

[attrs elided]

> > > I've done a lot of work with SAS disk enclosures that support SES. They 
> > > often have an SES command that can turn off the drive in a bay prior to 
> > > removal (but support is optional).
> >
> > Aren't standards *wonderful*?  What value to "optional" if folks can elect to
> > NOT implement??  <frown>
>
> All SES control requests are optional as some simple enclosures can only report 
> basic status. So its a genuinely has to be optional rather than a mandatory bit 
> of the standard everyone ignores. For power control the enclosure either has to 
> have dedicated drive power control hardware or initiate the stop unit commands 
> for itself which would have been challenging up until the SAS era. The 
> enclosures I worked on did support SES drive power :).

If I *have* to rely on this functionality (instead of just commanding the drive
to power down), then I'll have to do something to ensure the software doesn't
run on "just any old" server (and, sooner or later, someone will think about
using some desktop machine, etc.).  Maybe hardcode a test for the MAC in the
software?  Or, just serve everything up via PXE and control which machine
gets which software from there!

> > My read of the SATA spec indicated that hot plugging was part of the SATA
> > standard (and, by extension, SAS).  But, that support for it on HBA's was
> > (ahem) "optional".  In particular, support for cold presence detect (but, I'm
> > relying on the operator to perform that function as HE will be the person
> > doing the plugging/unplugging!)
>
> Anything AHCI will detect the removal. Not sure where netbsd is on 
> removing/adding disk devices now. Last time I tested it with SATA ahci was 7.1 
> and I couldn't force a freshly inserted disk to create a wd device but this 
> could be down to lack of knowledge on my part. AHCI did report the drive link 
> rate correctly when I plugged it in. drvctl looks like it ought to be able to 
> do the removal insertion but I'd need help from someone on the lists to advise 
> on how to make it work with disks ;).

I can impose some minimum constraints on the operator(s) who will be using
the fixture.  E.g., tell them to click on "spin down" before removing the
drive and "wait for the green light" before extracting.

[This last bit will be harder to count on as folks will get away with removing
the drive prematurely, once, and then infer that they can get away with it
ALWAYS -- as that helps them get more done in less time (so they can spend the
freed time playing with their telephone or...)  These aren't "IT" guys so
trying to impress upon them the *need* to do X, Y or Z is tough]

> > But, some bright-eyed dweeb uttered "Why not use an old server and write
> > something in Linux to do the job?!" -- because, to said dweeb, Linux is the
> > panacea for all problems technical (ignorance must be a wonderful state of
> > mind -- everything is "easy-peasy"!)
>
> Linux does have hotplug disk support (at least with SAS HBAs). Haven't tried 
> AHCI as all my AHCI systems are NetBSD. ;)

Ditto on the NetBSD point.  I'd rather stay out of the Linux swamp (at least in
my home lab).

I pulled a small SOHO server (PE840) out and put 7.1 on it so I can test on
that platform -- hopefully a bit closer to the servers at work.  It'll support
four drive sleds so I can try to get a feel for how/if all of this will work
without lugging the other server home (I hate rack-mounted servers around the
house).  Also give me a feel for how many drives I can process in parallel
and whether or not I can keep the operator "continuously busy".