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Re: Proposal: B_ARRIER (addresses wapbl performance?)

On Oct 29, 2008, at 6:57 PM, David Young wrote:

On Wed, Oct 29, 2008 at 05:51:09PM -0400, Thor Lancelot Simon wrote:
I'd like to propose a new buffer flag, B_ARRIER.  This flag would be
defined to cause a write barrier at whatever I/O request carries it.


I have two questions about this.

How can a disk driver re-order B_BARRIER blocks with respect to other
blocks on its queue? I.e., can a driver ever postpone writing a B_BARRIER
block until after it writes some !B_BARRIER blocks?

We've actually been down this path before. I had a private tree years ago that included B_BARRIER support in both the disk queue code and passed it down the scsipi layer to be turned into ordered tags. The only logical implementation is one that honors B_BARRER at all layers where queueing happens.

-- thorpej

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