On Wed, Dec 13, 2017 at 8:04 AM, David Steele <da...@pgmasters.net> wrote: > On 12/12/17 5:52 PM, Andres Freund wrote: >> On 2017-12-12 17:49:54 -0500, David Steele wrote: >>> >>> Including unlogged relations in base backups takes up space and is >>> wasteful >>> since they are truncated during backup recovery. >>> >>> The attached patches exclude unlogged relations from base backups except >>> for >>> the init fork, which is required to recreate the main fork during >>> recovery. >> >> >> How do you reliably identify unlogged relations while writes are going >> on? Without locks that sounds, uh, nontrivial? > > > I don't think this is an issue. If the init fork exists it should be OK if > it is torn since it will be recreated from WAL.
Yeah, I was just typing that until I saw your message. > If the forks are written out of order (i.e. main before init), which is > definitely possible, then I think worst case is some files will be backed up > that don't need to be. The main fork is unlikely to be very large at that > point so it doesn't seem like a big deal. As far as I recall the init forks are logged before the main forks. I don't think that we should rely on that assumption though to be always satisfied. >>> I decided not to try and document unlogged exclusions in the continuous >>> backup documentation yet (they are noted in the protocol docs). I would >>> like to get some input on whether the community thinks this is a good >>> idea. >>> It's a non-trivial procedure that would be easy to misunderstand and does >>> not affect the quality of the backup other than using less space. >>> Thoughts? >> >> >> Think it's a good idea, I've serious concerns about practicability of a >> correct implementation though. > > Well, I would be happy if you had a look! You can count me in. I think that this patch has value for some dedicated workloads. It is a waste to backup stuff that will be removed at recovery anyway. -- Michael
