On Wed, Mar 22, 2023 at 11:35 AM Melanie Plageman <melanieplage...@gmail.com> wrote: > > On Fri, Mar 17, 2023 at 8:51 PM Michael Paquier <mich...@paquier.xyz> wrote: > > > > On Fri, Mar 17, 2023 at 04:36:58PM -0700, Peter Geoghegan wrote: > > > I'm sure that they will do that much more than they would have > > > otherwise. Since we'll have made pg_get_wal_block_info() so much more > > > useful than pg_get_wal_records_info() for many important use cases. > > > Why is that a bad thing? Are you concerned about the overhead of > > > pulling in FPIs when pg_get_wal_block_info() is run, if Bharath's > > > patch is committed? That could be a problem, I suppose -- but it would > > > be good to get more data on that. Do you think that this will be much > > > of an issue, Bharath? > > > > Yes. The CPU cost is one thing, but I am also worrying about the > > I/O cost with a tuplestore spilling to disk a large number of FPIs, > > and some workloads can generate WAL so as FPIs is what makes for most > > of the contents stored in the WAL. (wal_compression is very effective > > in such cases, for example.) > > I had done some analysis about CPU costs for decompressing FPI upthread > in [1], finding that adding a parameter to allow skipping outputting FPI > would not have much impact when FPI are compressed, as decompressing the > images comprised very little of the overall time. > > After reading what you said, I was interested to see how substantial the > I/O cost with non-compressed FPI would be. > > Using a patch with a parameter to pg_get_wal_block_info() to skip > outputting FPI, I found that on a fast local nvme ssd, the timing > difference between doing so and not still isn't huge -- 9 seconds when > outputting the FPI vs 8.5 seconds when skipping outputting FPI. (with > ~50,000 records all with non-compressed FPIs). > > However, perhaps obviously, the I/O cost is worse. > Doing nothing but > > SELECT * FROM pg_get_wal_block_info(:start_lsn, :end_lsn, true) > where fpi is not null;
Sorry, I should have been more clear: similar results with a select list simply excluding fpi and no where clause. - Melanie
