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SubjectRe: [NBD] patch and documentation
>>The patch also looks harmless enough for applying ;-).
>
> Harmless enough, although I'm not sure it really makes that much
> difference. The max_sectors being set to 255 doesn't, by itself, explain
> the back and forth 127k, 1k request thing. Typically what you'll see is
> 127k, 127k, 127k, etc. and then some odd sized request at the end. Or
> the device gets unplugged anyway at some point and there are odd sized
> requests scattered throughout...that's especially going to be true if
> the reads or writes are from an actual disk, rather than /dev/null. I
> may be just coincidence that setting max_sectors to 256 actually helps.
> Also, are we sure that all those requests you're seeing are of the same
> type (all reads, all writes)?

Well, i guess the cache uses a value of 256 sectors to do read-ahead and
such. I used dd if=/dev/nbd/0 of=/dev/null bs=X with both X=1 and X=1M.
Both with the same result. That the 1byte requests join together to
bigger ones can only be explained with read-aheads strategies.
Anyway, the result is always the same:

without patch: 127KB, 1KB, 127KB, 1KB
with path: 128KB, 128KB, 128KB

As long as dd doesn't write i'm sure that i didn't see any write
requests. In addition it is a very regular pattern.
If it is really the case that the cache reads 256 sectors and the
default limit is 255, than this would also happen for all other
block-devices. In addition it would be a good thing to look up if the
cache takes the max_sectors stuff into accout while determining the
amout of sectors it reads ahead.


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