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. Regards, -- Michael -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
