On 4/12/22 17:46, Simon Riggs wrote: > On Tue, 12 Apr 2022 at 16:41, Tomas Vondra > <tomas.von...@enterprisedb.com> wrote: >> >> On 4/12/22 15:58, Simon Riggs wrote: >>> On Thu, 7 Apr 2022 at 08:46, Thomas Munro <thomas.mu...@gmail.com> wrote: >>> >>>> With that... I've finally pushed the 0002 patch and will be watching >>>> the build farm. >>> >>> This is a nice feature if it is safe to turn off full_page_writes. >>> >>> When is it safe to do that? On which platform? >>> >>> I am not aware of any released software that allows full_page_writes >>> to be safely disabled. Perhaps something has been released recently >>> that allows this? I think we have substantial documentation about >>> safety of other settings, so we should carefully document things here >>> also. >>> >> >> I don't see why/how would an async prefetch make FPW unnecessary. Did >> anyone claim that be the case? > > Other way around. FPWs make prefetch unnecessary. > Therefore you would only want prefetch with FPW=off, AFAIK. > > Or put this another way: when is it safe and sensible to use > recovery_prefetch != off? >
That assumes the FPI stays in memory until the next modification, and that can be untrue for a number of reasons. Long checkpoint interval with enough random accesses in between is a nice example. See the benchmarks I did a year ago (regular pgbench). Or imagine a r/o replica used to run analytics queries, that access so much data it evicts the buffers initialized by the FPI records. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
