Md Raid or Drbd Can Be Broken From Userspace When Using O_direct

indeed out of date - that was merged a long time ago and shipped in a stable version earlier this year.

The practical purpose of O_DIRECT is to have precise visibility and control over what is in memory, what is on disk, and any inflight I/O operations. This opens up an entire category of workload-aware execution scheduling optimizations that become crucial for performance as storage sizes increase.

O_DIRECT is used to disable cache replacement algorithms entirely in contexts where their NP-hardness becomes unavoidably pathological. You can't fix "fundamentally broken algorithm" with more knobs.

The canonical solution for workloads that break cache replacement is to dynamically rewrite the workload execution schedule in realtime at a very granular level. A prerequisite for this when storage is involved is to have perfect visibility and control over what is in memory, what is on disk, and any inflight I/O operations. The execution sequencing and I/O schedule are intertwined to the point of being essentially the same bit of code. For things like database systems this provides qualitative integer factor throughput improvements for many workloads, so very much worth the effort.

Without O_DIRECT, Linux will demonstrably destroy the performance of the carefully orchestrated schedule by obliviously running it through cache replacement algorithms in an attempt to be helpful. More practically, O_DIRECT also gives you fast, efficient visibility over the state of all storage the process is working with, which you need regardless.

Even if Linux handed strict explicit control of the page cache to the database process it doesn't solve the problem. Rewriting the execution schedule requires running algorithms across the internal page cache metadata. In modern systems this may be done 100 million times per second in userspace. You aren't gatekeeping analysis of that metadata with a syscall. The way Linux organizes and manages this metadata couldn't support that operation rate regardless.

Linux still needs to work well for processes that are well-served by normal cache replacement algorithms. O_DIRECT is perfectly adequate for disabling cache replacement algorithms in contexts where no one should be using cache replacement algorithms.