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I did not imply that O_DIRECT was a full equivalent of raw devices, it was rather a compromise as Linus would not concede with the idea of raw devices making it into Linux.

Besides, O_DIRECT can be used at the file level (and the file system level as well via chattr – if the file system supports it) whereas the raw devices are, well, device level only.

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I’m pretty sure you can open disks raw in Linux. But then the disk is owned by the RDBMS. Ceph can do this in its BlueStore engine.

But yeah, I understand you weren’t talking about buffers, I was just highlighting that O_DIRECT and raw devices are totally separate problems from having a user space cache that can be evicted by the kernel memory and reconstructed by the application lazily.

Yes, you can use raw block devices in Linux. LMDB works on raw block devices as well. You get write-through cache behavior; writes are immediate but everything still goes into the page cache.

I've not yet bothered to check how things work with raw devices in Windows. I believe only Admins can even operate on them so it would be kind of limited usefulness.

I don’t believe the page cache is accessed if you open the raw block device via o_direct.

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