Troubleshooting cache data corruption
This goes back a few years, but it's a condition
that is always possible. The symptoms were occasional
data corruption but only in frequently used files. All
the usual suspects were hauled out and examined; network
problems, application bugs, user error, hard disk sectors:
everything passed muster.
I was called in, and after listening to the sad tale,
I wrote a little shell script very similar to this:
cd /yourdatadir # change this to wherever your files are
echo "Flushing cache"
ls -lR /
sum * > /tmp/firstread
sum * > /tmp/a
sum * > /tmp/b
diff /tmp/a /tmp/b || break
echo "Corruption detected!"
echo "a vs. firstread"
diff /tmp/a /tmp/firstread
echo "b vs. firstread"
diff /tmp/b /tmp/firstread
echo "Flushing cache"
ls -lR /
sum * > /tmp/newread
echo "firstread vs. newread:"
diff /tmp/firstread /tmp/newread
The idea here is that data read from the disk should
always have the same sum (assuming a quiescent system). The
data files were small enough that all data read would be
cached, and the only thing that each "sum" would do
after the first is read from cache. Therefor, if there
was any change in the sums, cache would be the problem.
Indeed, after twenty minutes or so, the script exited,
announcing a difference.
As there was no difference betwwen "firstread" and "newread",
nothing had changed on the disk itself (unless it coincidentally
switched back; rather unlikely): cache definitely was looking very
guilty. But which cache? Was it the system buffer cache or the
raid controller? To determine that, I disabled the disk cache
(fortunately easy to do with that controller). The test was
repeated, and no errors were observed after an hour. I then
re-enabled the disk cache, and was able to repeat the
sum errors within a few minutes. That seemed to be pretty
definite proof of where the problem was, so the hardware was
replaced the following week and, as expected, the corruption
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