[dm-devel] 2.6.10-rc1-udm1: multipath work in progress

[Stefan Bader]

> The idea was (at least on my side) to minimize unnecessary switches
> between priority groups by the kernel itself, as this might have
> undesireable side effects. (Performance or affecting other nodes.) In
> short, to move the policy decision of when to switch the priority group
> out of the kernel to user-space.
> 
> The only time when the kernel was 'authorized' to switch PGs internally
> was in response to all paths failing or when the PG was "pulled away"
> from under us (in a non-fatal condition). In short, when immediate and
> unavoidable action was necessary to keep the IO moving.

Couldn't that goal be reached by simply giving path-groups a priority 
number?
The kernel would stick to the current pg as long there is no other pg with 
a
lower/higher priority or an all-path fault has occured on the current pg.
If you set all priorities to the same number there is no fallback. If you 
add
an message to change priority numbers any tool can change the behaviour 
and
even force to switch to any group.

/Stefan Bader