Re: [PATCH + RFC] Beginning of some updates to scsi mid layer

Matthew Jacob
2002-07-01T19:23:56Z

On Mon, 1 Jul 2002, Doug Ledford wrote:

> On Mon, Jul 01, 2002 at 12:08:03PM -0700, Matthew Jacob wrote:
> > Yes, exponential backoff is good in a lot of cases. BTW- this is something I
> > want to put into cam_periph_error for FreeBSD- I want to make the retry after
> > selection timeout also have exponential delays up to retry count.
>
> Hmmm...what's the purpose on this BTW? Selection timeouts on what, fiber
> or SPI or something else?

Fibre. Devices 'go away' for shortish to longish periods of time on Fibre (or
iSCSI for that matter). Lacking a sane framework that is USB-like about
hotpluggable that does *not* cause complete system spasm when this happens
(and it happens a lot), what one wants is a bit of hysteresis to give the
disks that went away a chance to log back into the fabric and thus appear
again (as if they had never gone).

>
> > In private mail with Doug && Pete, I managed to put myself into an untenable
> > position. The problem here is that we don't have end-to-end flow control, so
> > we have to *guess* when it might be good to try again.
>
> Yep. Experimentation can be your friend on that. That's what I did to
> come up with the alghorithm I was quoting. However, I didn't try
> exponential backoff so I make no claims that it wouldn't in fact be
> better.

But the trouble with experimentation can be like what got me into trouble in
our private email- I was relying on stale information (like, 10 years old) in
my thinking. Bad matt.....