The Linux kernel has reached a level of maturity where it mostly goes unnoticed and acts like an "invisible magician in the background", according to Linus Torvalds.
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"I think the pressure has been off the kernel for a long time now because a lot of the new features have been about userland and most people haven't even noticed the kernel.
"The kernel should be pretty much the invisible magician in the background -- unless things go wrong and hopefully they don't," said Torvalds.
When asked of the kernel has hit a level of maturity, he said: "We are still working on a lot of stuff and especially new hardware. On the whole a lot of the basics are there. What we work on is better maintainability, improving the code so that we can add features more easily and occasionally adding a feature that some people care about but most will not even notice."
Having recently moved a machine from 2.6.8 to 2.6.22 I've seen:
md_mod breakage (panics when using `internal' write-intent bitmap on raid1)
lvm snapshot breakage (deadlocks with muliple readers on the base volume of a snapshot that's filled up and been invalidated).
... and who knows what's to come.
Both issues to be reported to LKML once some more testing, panic-capturing, and writing-up is done. What troubles me is that I'm using a really ordinary md_mod+lvm configuration; this isn't anything special.
It's not hardware. The OS was cloned to alternate hardware (different mb model and chipset, different cpu, different SATA controller, different disks, etc) and had the same symptoms.
I always thought the 2.4 series was rather more stable, and recent experience seems to bear that out. I'm not comfortable with the risks that appear to be involved in kernel upgrades at the moment.
Maybe Andrew Morton is right and lots of bugs are just going unseen, subsystems and features untested, etc?