This is one of several examples where Linus thinks something is bad because he doesn't understand how it is used.

Something like O_DIRECT is critical for high-performance storage in software for well-understood reasons. It enables entire categories of optimization by breaking a kernel abstraction that is intrinsically unfit for purpose; there is no way to fix it in the kernel, the existence of the abstraction is the problem as a matter of theory.

As a database performance enjoyer, I've been using O_DIRECT for 15+ years. Something like it will always exist because removing it would make some high-performance, high-scale software strictly worse.

Of course you should offer some method to disable caching/compression/encryption/ECC/etc. in intermediate layers whenever those are non-zero cost and might be duplicated at application level... that's the ancient end-to-end argument.

But that method doesn't necessarily have to be "something like O_DIRECT", which turns into a composition/complexity nightmare all for the sake of preserving the traditional open()/write()/close() interface. If you're really that concerned about performance, it's probably better to use an API that reflects the OS-level view of your data, as Linus pointed out in this ancient (2002!) thread:

https://yarchive.net/comp/linux/o_direct.html

Or, as noted in the 2007 thread that someone else linked above, at least posix_fadvise() lets the user specify a definite extent for the uncached region, which is invaluable information for the block and FS layers but not usually communicated at the time of open().

I think it's quite reasonable to consider the real problem to be the user code that after 20 years hasn't managed to migrate to something more sophisticated than open(O_DIRECT), rather than Linux's ability to handle every single cache invalidation corner case in every possible composition of block device wrappers. It really is a poorly-thought-out API from the OS implementor's perspective, even if at first seemingly simple and welcoming to an unsophisticated user.

O_DIRECT isn't about bypassing the kernel for the sake of reducing overhead. The gains would be small if that was the only reason.

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.

His lack of industry experience is the root cause of many issues in Linux.

Although this is somewhat true, I think the bigger issue is expecting Linux to support all these use cases. Even if Linus accepted all use cases, it's a different story to maintain a kernel/OS that supports them all. The story from an engineering standpoint is just too unwieldy. A general-purpose OS can only go so far to optimize countless special-purpose uses.

This is not some minor niche use case though, and all other operating systems seem to have no trouble supporting OS fscache bypass.

Considering how big Linux is and how many different use cases it supports, this could well be an undue maintenance burden for Linux where it wouldn't be for other operating systems. Though, I'll grant that I don't know the details here, and of course Linus is...opinionated.

I agree. I wish we had more varied operating systems.

I asked my question in the wrong place!

"So is the original requirement for O_DIRECT addressed completely by O_SYNC and O_DSYNC"

I'm guessing you'd say "no"

O_DIRECT is separate from synchronization. There is no guarantee that O_DIRECT writes are durable, though a subset of hardware may work this way in fact.

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.

So is the original requirement for O_DIRECT addressed completely by O_SYNC and O_DSYNC?

The way I was told it, if the database engine implements it's own cache (like InnoDB and presumably Oracle), are just "doubling up" if you also use the OS cache?. Perhaps Oracle is happy with its own internal caching (for reads).

I've seen a DB guy insist on O_DIRECT without implementing array controller cache battery alerting, or checking if drives themselves had caches disabled. Nope "O_DIRECT fixes everything!" ... although these days enterprise class SSDs have little batteries and capacitors to handle power loss so in the right circumstances that's kinda resolved too, but like the array controller cache batteries, this is one more thing you have to monitor if you're running your own hardware