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Re: Proposal: B_ARRIER (addresses wapbl performance?)

On Dec 9, 2008, at 1:01 PM, Thor Lancelot Simon wrote:

On Tue, Dec 09, 2008 at 12:56:27PM -0800, Jason Thorpe wrote:

On Dec 9, 2008, at 12:23 PM, Manuel Bouyer wrote:

Sure. And you don't care much about command completion either, as
long as the write to disk happen in order.

And the only way to ensure that is using FUA (or explicit cache

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. So use FUA and wait for the journal writes to complete BEFORE ISSUING the other writes.

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. You can get consistency and good performance if you manage the cache on a per- command basis.


-- thorpej

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