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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.

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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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