Re: Hot Backup with rsync fails at pg_clog if under load

On Sep23, 2011, at 18:03 , Magnus Hagander wrote:
> On Sep 23, 2011 5:59 PM, "Alvaro Herrera" <alvherre@commandprompt.com> wrote:
> > Sounds like rsync is caching the file size at the start of the run, and
> > then copying that many bytes, ignoring the growth that occurred after it
> > started.
>
> That pretty much matches what Daniel does when he got the same failure case.
>
> Is this not allowed? Shouldn't wal replay fix this?

I don't see how it could be forbidden. ISTM that we absolutely *have* to be
able to deal with each byte of a file's date, including it's meta-data, being in
any state it was between the time pg_start_backup() returned and pg_stop_backup()
was issued. With the individual states being in no way synchronized.
(OK, in reality restricting this to individual 512-byte blocks is probably enough,
but still...).

This, I think, is also the reasons we need to force full_page_writes to on
during a hot backup. If a page was modified at all between pg_start_backup() and
pg_stop_backup(), we essentially have to assume it's totally garbled.

best regards,
Florian Pflug