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Re: Proposal: B_ARRIER (addresses wapbl performance?)

On Dec 9, 2008, at 11:00 AM, Manuel Bouyer wrote:

But I don't get why we want to have the journal on stable storage *now*.
From what I understand, what we want is to have the journal entry for
this transaction to stable storage *before* the metadata changes start to hit
stable storage, and the journal cleanup hits stable storage *after*
all metadata changes are on stable storage. I can't see what FUA brings
us here, as long as writes to stable storage are properly ordered.

You want your journal to remain self-consistent, otherwise you can't trust it to replay it.

Also, without explicit cache management, you can't be sure when the data gets written out of the drive's cache. Again, command completion has nothing to do with how the writes are ordered to the oxide.

-- thorpej

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