Re: WAL consistency check facility

Hello,

I've added the updated the patch with the necessary documentation and comments.
I've referenced Robert's reply in this thread and Simon's reply in
Production block comparison facility thread to write the documentation.

This feature is used to check the consistency of WAL records, i.e,
whether the WAL records are inserted and applied correctly.
A guc parameter named wal_consistency is added to enable this feature.
When wal_consistency is enabled for a WAL record, it stores a full-page image
along with the record. When a full-page image arrives during redo, it compares
against the current page to check whether both are consistent.

The default value for this setting is none. To check all records written to the
write-ahead log, set this parameter to all. To check only some records, specify
a comma-separated list of resource managers. The resource managers which
are currently supported are xlog, heap2, heap, btree, hash, gin, gist, spgist,
sequence, brin and generic.

If any inconsistency is detected, it throws a WARNING. But, as per discussions
in the earlier threads, it can be changed to ERROR./FATAL(just a one
word change).
I've kept this as warning because of some inconsistency in BRIN VACUUM
during gmake check.

In recovery tests, I've enabled this feature in PostgresNode.pm.

Thanks to Amit, Dilip, Michael, Simon and Robert for their valuable feedbacks.

Thoughts?

--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com