Bruce Evans 9e5ed8593f Use VOP_ADVISE() with POSIX_FADV_DONTNEED instead of IO_DIRECT to ... implement not double-caching for reads from vnode-backed md devices. Use VOP_ADVISE() similarly instead of !IO_DIRECT unsimilarly for writes. Add a "cache" option to mdconfig to allow changing the default of not caching. This depends on a recent commit to fix VOP_ADVISE(). A previous version had optimizations for sequential i/o's (merge the i/o's and only uncache for discontiguous i/o's and for full blocks), but optimizations and knowledge of block boundaries belong in VOP_ADVISE(). Read-ahead should also be handled better, by supporting it in md and discarding it in VOP_ADVISE(). POSIX_FADV_DONTNEED is ignored by zfs, but so is IO_DIRECT. POSIX_FADV_DONTNEED works better than IO_DIRECT if it is not ignored, since it only discards from the buffer cache immediately, while IO_DIRECT also discards from the page cache immediately. IO_DIRECT was not used for writes since it was claimed to be too slow, but most of the slowness for writes is from doing them synchronously by default. Non-synchronous writes still deadlock in many cases. IO_DIRECT only has a special implementation for ffs reads with DIRECTIO configured. Otherwise, if it is not ignored than it uses the buffer and page caches normally except for discarding everything after each i/o, and then it has much the same overheads as POSIX_FADV_DONTNEED. The overheads for reading with ffs and DIRECTIO were similar in tests of md. Reviewed by: kib		2018-12-21 08:15:31 +00:00
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tests
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Makefile.depend
mdconfig.8	Use VOP_ADVISE() with POSIX_FADV_DONTNEED instead of IO_DIRECT to	2018-12-21 08:15:31 +00:00
mdconfig.c	Use VOP_ADVISE() with POSIX_FADV_DONTNEED instead of IO_DIRECT to	2018-12-21 08:15:31 +00:00