Re: [PATCHES] Disable page writes when fsync off, add GUC

Stephen Frost wrote:
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> * Bruce Momjian (pgman(at)candle(dot)pha(dot)pa(dot)us) wrote:
> > This patch disables page writes to WAL when fsync is off, because with
> > no fsync guarantee, the page write recovery isn't useful.
>
> This doesn't seem quite right to me. What happens with PITR? And

PITR doesn't need page writes at all because it has a full backup the
file system to start with. In fact with PITR the crashed file system
isn't used at all (restored from backup). In fact there is a TODO to
exclude full page writes from the PITR backup of WAL.

> Postgres crashes? While many people seriously distrust running w/ fsync
> off, I'm sure there's quite a few folks which do.
>
> > This also adds a full_page_writes GUC to turn off page writes to WAL.
> > Some people might not want full_page_writes, but still might want fsync.
>
> Adding an option to not do page writes to WAL seems fine to me, but I
> think WAL writes should be on by default, even in the fsync=off case.
> If people want to turn it off, fine, for either case since we expect
> they understand what it means to have it turned off, but I don't think
> the two options should be coupled as is being proposed.

That is a question I had in my mind. I added documentation that turning
off fsync also disables full_page_writes, but we could decouple them and
tell people to consider disableing full_pages_writes if they turn off
fsync, basically suggesting they make the second change.

Other opinions?

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