Messages in this thread | | | Date | Mon, 08 Sep 2003 22:17:42 +0200 | From | Sven Köhler <> | Subject | Re: [NBD] patch and documentation |
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>>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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