Phillip Susi wrote:
Hrm... I may have misunderstood the perspective you were talking from.
Yes, when the bio is completed it must be on the media, but the
filesystem should issue both requests, and then really not care when
they complete. That is to say, the filesystem should not wait for block
A to finish before issuing block B; it should issue both, and use
barriers to make sure they hit the disk in the correct order.
Actually now that I think about it, that wasn't correct. The request
CAN be completed before the data has hit the medium. The barrier just
constrains the ordering of the writes, but they can still sit in the
disk write back cache for some time.
Stefan Bader wrote:
That would be the exactly how I understand Documentation/block/barrier.txt:
"In other words, I/O barrier requests have the following two properties.
1. Request ordering
...
2. Forced flushing to physical medium"
"So, I/O barriers need to guarantee that requests actually get written
to non-volatile medium in order."
I think you misinterpret this, and it probably could be worded a bit
better. The barrier request is about constraining the order. The
forced flushing is one means to implement that constraint. The other
alternative mentioned there is to use ordered tags. The key part there
is "requests actually get written to non-volatile medium _in order_",
not "before the request completes", which would be synchronous IO.
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