With O_DIRECT though you're opting out of the filesystem's caching (well, VFS's), forced flushes, and most FS level optimizations, so I'd expect it to perform on par with direct partition access.
Do you have numbers showing an advantage of going directly to the block device? Personally, I'd consider the management advantages of a filesystem compelling absent specific performance numbers showing the benefit of direct partition access.
You do when it does that/respects it which isn’t always. The point is that you have more layers. If you’re trying to be as direct as possible, more layers is unhelpful.
Since you get most of the same advantages management wise with lvm while using the block interface (including snapshots, resizing, and all the other management goodies), you’re not exactly getting much extra functionality either.
Your concerns are all theoretical and the management disadvantages of direct partition access are real with or without LVM (which itself is exactly the sort of middle layer you claim to be worried about.)
Since most of what we’re talking about is unnecessary complexity for no real gain, what concrete metric do you think would be useful exactly? I just pointed out that you can get the same management advantages without it (say for a dev environment or rollbacks or whatever). And you get a simpler, cleaner story without extra layers if you don’t want to use lvm (such as in production), which you can’t get from O_DIRECT.
