Re: reiser4 plugins

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Theodore Ts'o wrote:
On Mon, Jun 27, 2005 at 03:18:30PM -0500, Steve Lord wrote:

I presume Ted is referring to problems guaranteeing the integrity of
the journal at recovery time. I am coming into this without all the
available context, so I may be barking up the wrong tree.... In
particular, I am not sure how journaling whole blocks protects
you from this.


Actually, I was talking about the problem what happens when power
fails while DMA'ing to the disk, and memory, which is more sensitive
to voltage drops than the rest of the system, starts sending garbage
to the bus, which the disk then faithfully writes to the inode table.

As I recall, you were the one who told me about this problem, and how
it was fixed in Irix by using a powerfail interrupt to abort DMA
transfers, as well as giving me a program which tests for this
condition (basically it writes known test pattern to the disk, and
then you do an unclean shutdown, and you look to see if garbage is
written to the disk instead of one of the known test patterns).  If it
wasn't you, it must have been Jim Mostek --- but I could have sworn it
was you.....

						- Ted


Your memory is better than mine, not sure about the test program, but
there was at one point a scenario like that on Irix, and I quite
probably did mention it to you. That was certainly not PC hardware,
but more likely a large Irix system with multiple power busses spread
around a large amount of hardware. I presume you are saying that the fact
that ext3 journals complete inode blocks rather that subsections of inodes
means that, should a similar trashing of metadata occur during power down,
a journal replay would fix it?

I see no way short of hardware fixes of avoiding the general problem of
a system failing in an ugly manner like this. Unless you write everything
to disk twice (i.e. journal all data), you can still end up with a
legitimate set of metadata, and the master copy of your employee
database full of nasty little bits of corruption.

Steve
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