On Thu, Dec 4, 2014 at 8:37 PM, Michael Paquier <michael.paqu...@gmail.com> wrote: > On Thu, Dec 4, 2014 at 7:36 PM, Rahila Syed <rahilasyed...@gmail.com> wrote: >> IIUC, forcibly written fpws are not exposed to user , so is it worthwhile to >> add a GUC similar to full_page_writes in order to control a feature which is >> unexposed to user in first place? >> >> If full page writes is set 'off' by user, user probably cannot afford the >> overhead involved in writing large pages to disk . So , if a full page write >> is forcibly written in such a situation it is better to compress it before >> writing to alleviate the drawbacks of writing full_page_writes in servers >> with heavy write load. >> >> The only scenario in which a user would not want to compress forcibly >> written pages is when CPU utilization is high. But according to measurements >> done earlier the CPU utilization of compress='on' and 'off' are not >> significantly different. > > Yes they are not visible to the user still they exist. I'd prefer that we have > a safety net though to prevent any problems that may occur if compression > algorithm has a bug as if we enforce compression for forcibly-written blocks > all the backups of our users would be impacted. > > I pondered something that Andres mentioned upthread: we may not do the > compression in WAL record only for blocks, but also at record level. Hence > joining the two ideas together I think that we should definitely have > a different > GUC to control the feature, consistently for all the images. Let's call it > wal_compression, with the following possible values: > - on, meaning that a maximum of compression is done, for this feature > basically full_page_writes = on. > - full_page_writes, meaning that full page writes are compressed > - off, default value, to disable completely the feature. > This would let room for another mode: 'record', to completely compress > a record. For now though, I think that a simple on/off switch would be > fine for this patch. Let's keep things simple.
+1 Regards, -- Fujii Masao -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers