On 11/24/2014 01:45 PM, Zach Brown wrote:
On Fri, Nov 21, 2014 at 05:00:31PM -0500, Josef Bacik wrote:
Hello,
I'm hoping some FS guys can weigh in and verify my approach for testing power
fail conditions, and the DM guys to of course verify I didn't completely fail at
making a DM target. All suggestions welcome, I want to have a nice robust tool
for testing power fail so we can be sure our fsync/commit code is all working
properly. Thanks,
*All* suggestions welcome? OK, I'll put on my skeptic hat.
This implements a writeback cache in kernel data structures so that you
can race to throw away cached blocks that haven't been flushed. How is
that meaningfully different than using an actual writeback caching dm
target and racing to invalidate it?
I didn't think of the dm-cache target, but do we want to add data loss
testing code to something people actually use in production? I feel
like that's a recipe for disaster. I suppose it could work, but my
target adds some specific scenarios like blow up after FUA/FLUSH to test
for specific races.
Using real caching dm target configurations would let you reuse their
testing and corner case handling that is, presumably, already slightly
more advanced than printk() swearing.
Well that's just an unfair jab, I missed _one_ debug printk.
Implementation of this specific mechanism aside, the architectural
observation is that device ram disappearing during a power interruption
is just one, and possibly the most forgiving, outcome. Pulling the
power out from under concurrnet cached writes can result in any
combination of them being written, or not, or scrambled to the point of
returning read errors.
So that's what this target tries to get around, we don't care about what
happens to the data that is outstanding, we act like it never happened,
because if we are going to rely on reading that data at all then we've
already lost. So simply acting like it disappeared is just as bad as it
being garbage or returning an EIO (though returning an EIO would be
pretty cool too and easily be added to the target).
If we were to justify developing a specific power failure target, I'd
like to see something that tracks write history and can replay the
history to offer a resonably exhaustive set of possible write results.
Verify *those* and you have much more confidence that the file system
can handle reading the results of its interrupted writes.
This sounds like a pretty cool idea, it would be weird trying to order
everything out though to catch problems where we don't properly wait on
IO to complete before we do flushing. You'd probably have to keep track
of when things were submitted and when they completed in the log in
order to replay them in a way to expose problems with the flushing. But
you're right it would allow us to more exhaustively test all different
scenarios. Thanks,
Josef
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