Re: pg_rewind - enable wal_log_hints or data-checksums

On Mon, Jun 5, 2017 at 9:37 AM, Dylan Luong <Dylan(dot)Luong(at)unisa(dot)edu(dot)au> wrote:
> pg_rewind requires that the target server either has the wal_log_hints
> option enabled in postgresql.conf or data checksums enabled when the cluster
> was initialized with initdb.

Yes, this is to make sure that you don't finish with a corrupted
target server if a hint bit is set on a page after a checkpoint. Any
of those options make sure that a full-page write is generated in this
case.

> What is the difference between the two options?

Data checksums calculate 2 bytes of checksum data and write it to each
page that is evicted from shared buffers. Each page read from disk has
its checksum checked. In some workloads, like a heavy read load where
a lot of page evictions happen, this can induce a couple of percents
of performance lost. In my own experience, that's 1~2%.

> What are the advantages and disadvantages between the two?
> Which one is the the preferred option?

If you care more about performance or if you use a file system that
has its own block-level checksum, wal_log_hints would be preferred.
Data checksums offer more guarantees in terms of integrity though when
looking for corrupted data. Things get found more quickly.
--
Michael