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Before Direct I/O was implemented, I've implemented lighter version I called Uncached I/O. It uses normal DMU/ARC data path with some optimizations, but evicts data from caches as soon as possible and reasonable. Originally I wired it only to a primarycache property, but now completing the integration all the way up to the VFS. While Direct I/O has the lowest possible memory bandwidth usage, it also has a significant number of limitations. It require I/Os to be page aligned, does not allow speculative prefetch, etc. The Uncached I/O does not have those limitations, but instead require additional memory copy, though still one less than regular cached I/O. As such it should fill the gap in between. Considering this I've disabled annoying EINVAL errors on misaligned requests, adding a tunable for those who wants to test their applications. To pass the information between the layers I had to change a number of APIs. But as side effect upper layers can now control not only the caching, but also speculative prefetch. I haven't wired it to VFS yet, since it require looking on some OS specifics. But while there I've implemented speculative prefetch of indirect blocks for Direct I/O, controllable via all the same mechanisms. Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored by: iXsystems, Inc. Fixes #17027 Reviewed-by: Rob Norris <robn@despairlabs.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> |
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