Online verification of checksums

Hi,

v10 almost added online activation of checksums, but all we've got is
pg_verify_checksums, i.e. offline verification of checkums. 

However, we also got (online) checksum verification during base backups,
and I have ported/adapted David Steele's recheck code to my personal
fork of pg_checksums[1], removed the online check (for verification) and
that seems to work fine.

I've now forward-ported this change to pg_verify_checksums, in order to
make this application useful for online clusters, see attached patch.

I've tested this in a tight loop (while true; do pg_verify_checksums -D
data1 -d > /dev/null || /bin/true; done)[2] while doing "while true; do
createdb pgbench; pgbench -i -s 10 pgbench > /dev/null; dropdb pgbench;
done", which I already used to develop the original code in the fork and
which brought up a few bugs.

I got one checksums verification failure this way, all others were
caught by the recheck (I've introduced a 500ms delay for the first ten
failures) like this:

|pg_verify_checksums: checksum verification failed on first attempt in
|file "data1/base/16837/16850", block 7770: calculated checksum 785 but
|expected 5063
|pg_verify_checksums: block 7770 in file "data1/base/16837/16850"
|verified ok on recheck

However, I am also seeing sporadic (maybe 0.5 times per pgbench run)
failures like this:

|pg_verify_checksums: short read of block 2644 in file
|"data1/base/16637/16650", got only 4096 bytes

This is not strictly a verification failure, should we do anything about
this? In my fork, I am also rechecking on this[3] (and I am happy to
extend the patch that way), but that makes the code and the patch more
complicated and I wanted to check the general opinion on this case
first.


Michael

[1] https://github.com/credativ/pg_checksums/commit/dc052f0d6f1282d3c821
5b0eb28b8e7c4e74f9e5
[2] while patching out the somewhat unhelpful (in regular operation,
anyway) debug message for every successful checksum verification
[3] https://github.com/credativ/pg_checksums/blob/master/pg_checksums.c#
L160
-- 
Michael Banck
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