Subject: Re: UBC status
To: Thor Lancelot Simon <email@example.com>
From: Matthew Jacob <firstname.lastname@example.org>
Date: 09/27/1999 09:25:06
On Mon, 27 Sep 1999, Thor Lancelot Simon wrote:
> On Mon, Sep 27, 1999 at 08:45:20AM -0700, Matthew Jacob wrote:
> > >
> > > > Solaris has a B_ORDERED flag which is a hint specifically for this type
> > > > of thing.
> > > >
> > > > Basically, metadata writes (and any other writes that need to be ordered)
> > > > would set B_ORDERED, and this would affect disksort() (for disks which
> > > > don't supported tagged commands) and the type of tag used (for disks that do).
> > > >
> > > > A B_URGENT (Head-Of-Queue tag) flag might also be useful.
> > >
> > > I don't believe that any queue types other than simple queue are used,
> > > primarily due to buggy implementations of other tag types in target
> > > devices. You must be thinking of some other OS.
> > Solaris uses FLAG_STAG, yes. Kleiman originally wanted B_ORDERED to be
> > propagated to the driver so that a FLAG_OTAG would then be used to force
> > all the previous STAG operations out. I guess this didn't get followed up
> > on.
> What is the point of having a B_ORDERED flag if the target device is,
> ultimately, allowed not to treat it as a barrier?
Because the *filesystem* or the *driver* can do the barrier.
> Certainly, to get the FFS metadata semantics right, AFAICT, you really
> want to use an ordered tag to force all the previous I/O you've issued to
> the disks to complete. This is why we currently use ordered tags for
> *all* writes in at least some drivers (any that don't should!). At least
Yeah! I noticed that I'd putzed this up until recently in the isp driver!
> one Linux device driver I've looked at closely does what Jason's proposing:
> use ordered tags as barriers to force simple tags to complete.
Jason's proposing the right thing.
> How can Solaris preserve FFS metadata consistency guarantees if it
> writes using simple tags?
Got me on that one. Eduardo?