Write barriers

Write barriers

Posted Jul 14, 2006 19:15 UTC (Fri) by giraffedata (subscriber, #1954)
In reply to: Three comments by nix
Parent article: Crash-only software: More than meets the eye

No, the problem that write barriers solve is where the device says "OK, I've got the data" and Linux considers the data to be permanent based on that. Before write barriers, that's what happens.

That's not ridiculous, by the way. "Permanent" is a matter of degree, and being written on the platter is just one degree in the middle of the scale. Once the device has the data, it is safe from a Linux kernel crash, and that's a lot.

Write barriers are, BTW, a Linux kernel block layer phenomenon; the device doesn't know the concept. Linux has various ways to know that the device has put the data on the platter and uses them to implement write barriers. But if the device lies, the write barriers won't work.

Since the device lies to circumvent a system that explicitly asked for the data to go on the platter, I rather doubt that it would refrain from lying when Linux write barriers are involved.

BTW, I can't confirm or deny that devices lie like this, and to the extent claimed. If someone can back up this claim, I'd love to see it.

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