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Re: Proposal: B_ARRIER (addresses wapbl performance?)

On Tue, Dec 09, 2008 at 10:41:47PM +0100, Manuel Bouyer wrote:
> On Tue, Dec 09, 2008 at 01:30:10PM -0800, Jason Thorpe wrote:
> > >But to enforce that using FUA, you have to set FUA on *every* write.
> > 
> > The only thing you have to ensure is that journal is consistent and on- 
> > disk before the other metadata blobs.
> I don't think it's so simple. I think there's also metadata writes that
> needs to happen *before* journal writes.

The only place I see FUA on non-journal writes would matter would be to
make it easier to roll the journal.

> > So use FUA and wait for the  
> > journal writes to complete BEFORE ISSUING the other writes.
> OK, so you use software barrier, not the ones provided by disks
> (this is what was not clear).
> But I think with this we'll end up issuing mostly FUA writes, because
> ordering constraint are both way. So it won't be any better than
> running simple queue tags with the write cache disabled (what we're
> doing now).

I think the advantage of FUA over write cache off is if we aren't 
journaling file data. If we are journaling everything, then write-cache 
off + simple tags will suffice. But my experience is to just journal 
fs metadata, so the difference is important.

> > >And I do not understand the "only way" in your remark above.  Under
> > >the constraint "if WCE is set in the cache control page" I agree that
> > >it is correct.  But as far as I can tell, if WCE is *not* set, it is
> > >not in fact the case that using FUA is the "only way" to ensure that
> > >writes are committed to stable storage in order -- because the tag
> > >ordering rules require ordered tags to complete, um, well, in-order,
> > >and if WCE is not set, commands are not supposed to complete until
> > >the bits are on oxide.
> > >
> > >I am sure I am misunderstanding something.  What?
> > 
> > Turning off the write cache makes things too bloody slow.
> With tagged queuing and simple queue tag it's not my experience.

It is however the experience of others.

Take care,


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