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SubjectRe: Soft-Updates for Linux ?
On Sun, Oct 01, 2000 at 10:21:34AM -0500, Robert Redelmeier wrote:

> As an experiment, I pulled the plug towards the end of 5 FreeBSD kernel
> compiles (SMP `make -j4`). In all cases, the fsck upon restart was minor,
> just freeing inodes. In four of the cases, `make` just picked up where
> it left off, and finished the kernel compile, losing only ~40 seconds work!
> In one case, a `make clean` had to be done because something was incomplete.
> You boys and girls don't try this at home on Linux! The ext2 fsck is horrible
> after a powerfail, and I've lost superblocks and had to re-install :( .

There is actually some indication that part of the corruption people observe
after powerfails is caused by hardware scribbling junk data over the disk
in the very last moment when power levels in the system are already so low
that memory is already starting to fail while I/O hardware is still doing
their best to write junk data to the disk. These days powerfails are rare
and so it's easy to attribute that to something else.

I'm now using ext2 since '93 or so and I haven't ever lost a complete
filesystem. In fact in practice it's amazing how resistant ext2 is to
extreme data corruption such as caused by the good old problem of the kernel
and ext2utils having different ideas of the bitfield endianess ...

Maybe it's time to implement support for power fail / power return interrupts
where supported by hardware. At least on Indy that gives you ~ 10ms to
stop whatever is going on before power finally fails.

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