Re: reiser4 plugins

Steve Lord
2005-06-27T20:59:41Z

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