geom_sched influence on SU consistency

[Luigi Rizzo]

On Wed, May 5, 2010 at 3:24 PM, Dominic Fandrey <kamikaze at bsdforen.de> wrote:
> I'm wondering how geom_sched influences soft-update consistency.
>
> To my understanding it's very important to SU, that the file system
> controls writing sequences. Because geom_sched is transparent,
> i.e. UFS does not know about access scheduling, I'm afraid that
> the use of geom_sched would endanger my file system consistency
> in case of a crash.
>
> Can anyone put my fears to rest or confirm them?

geom_sched is no different from the standard (elevator)
disk scheduler or tagged queueing -- these system may already serve
queued requests out of order.

if a client wants to enforce a specific ordering of requests it can
easily do so by waiting the completion of a previous request before
issuing a new one.