On Friday 05 November 2010 22:10:36 Greg Smith wrote: > Andres Freund wrote: > > On Sunday 31 October 2010 20:59:31 Greg Smith wrote: > >> Writes only are sync'd out when you do a commit, or the database does a > >> checkpoint. > > > > Hm? WAL is written out to disk after an the space provided by > > wal_buffers(def 8) * XLOG_BLCKSZ (def 8192) is used. The default is 64kb > > which you reach pretty quickly - especially after a checkpoint. > Fair enough; I'm so used to bumping wal_buffers up to 16MB nowadays that > I forget sometimes that people actually run with the default where this > becomes an important consideration. If you have relatively frequent checkpoints (quite a sensible in some environments given the burstiness/response time problems you can get) even a 16MB wal_buffers can cause significantly more synchronous writes with O_DSYNC because of the amounts of wal traffic due to full_page_writes. For one the background wal writer wont keep up and for another all its writes will be synchronous... Its simply a pointless setting. > > Not having a real O_DSYNC on linux until recently makes it even more > > dubious to have it as a default... > If Linux is now defining O_DSYNC, and it's buggy, that's going to break > more software than just PostgreSQL. It wasn't defined before because it > didn't work. If the kernel developers have made changes to claim it's > working now, but it doesn't really, I would think they'd consider any > reports of actual bugs here as important to fix. There's only so much > the database can do in the face of incorrect information reported by the > operating system. I don't see it being buggy so far. Its just doing what it should. Which is simply a terrible thing for our implementation. Generally. Independent from linux. > Anyway, I haven't actually seen reports that proves there's any problem > here, I was just pointing out that we haven't seen any positive reports > about database stress testing on these kernel versions yet either. The > changes here are theoretically the right ones, and defaulting to safe > writes that flush out write caches is a long-term good thing. I have seen several database which run under 2.6.33 with moderate to high load for some time now. And two 2.6.35. Loads of problems, but none kernel related so far ;-) Andres -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
