Yes. The O_DIRECT issue is my biggest concern about Linux at the moment.
That being said, the plan is to only pre-fetch the next N blocks, where N
< 32, and to read them into the local buffer cache. In a situation where
space in the cache low (and prefetched pages might be pushed out before we
even get to read them), we need to provide such information to the
readahead mechanism so that it can reduce the number of blocks which it
prefetches.
Would you open a separate handle O_DIRECT, just for the prefetch ?
My experience with O_DIRECT and databases in the past has not been
great : what you gain with being able to control your own caching you loose
(and more) in other ways.
BTW, has anyone tried O_DIRECT and the prefetch idea on Linux ?
I'm wondering if it may not work (because the read data won't get cached
in the fs cache due to O_DIRECT).
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