Subject: Re: wd, disk write cache, sync cache, and softdep.
To: None <>
From: J Chapman Flack <>
List: tech-kern
Date: 12/16/2004 15:22:38
> > But under what conditions should higher layers set this flag?
> Metadata updates.
> If the upper level really cares about ordering, it should set this flag.

I've often wondered how the implications of hardware features like
write-back disk caches get addressed in higher software layers than
the OS code itself, such as the write-ahead log protocol in a database
engine.  My long-standing suspicion has been that I would really rather
not know; I'm probably lucky if I'm using an OS that is really enforcing
what it thinks it's enforcing in filesystem integrity, and it's probably
more like a miracle if some DBMS I've installed is really
doing the right things needed to get its WALP updates all the way down
to the media.  But I don't really know the state of the art.  Do all
transactional DBMSs have an inescapably OS-specific layer of logging
code where for NetBSD it would set this flag, and for another OS do
something else?  Is there any common practice existing or emerging that
we could track?