On Sat, Jun 5, 2010 at 6:07 PM, Jon Schewe <jpschewe@xxxxxxx> wrote: > On 06/05/2010 07:02 PM, Scott Marlowe wrote: >> On Sat, Jun 5, 2010 at 5:58 PM, Jon Schewe <jpschewe@xxxxxxx> wrote: >> >>> On 06/05/2010 06:54 PM, Scott Marlowe wrote: >>> >>>> On Sat, Jun 5, 2010 at 5:03 PM, Jon Schewe <jpschewe@xxxxxxx> wrote: >>>> >>>> >>>>> On 06/05/2010 05:52 PM, Greg Smith wrote: >>>>> >>>>> >>>>>> Jon Schewe wrote: >>>>>> >>>>>> >>>>>>>> If that's the case, what you've measured is which filesystems are >>>>>>>> safe because they default to flushing drive cache (the ones that take >>>>>>>> around 15 minutes) and which do not (the ones that take >=around 2 >>>>>>>> hours). You can't make ext3 flush the cache correctly no matter what >>>>>>>> you do with barriers, they just don't work on ext3 the way PostgreSQL >>>>>>>> needs them to. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> So the 15 minute runs are doing it correctly and safely, but the slow >>>>>>> ones are doing the wrong thing? That would imply that ext3 is the safe >>>>>>> one. But your last statement suggests that ext3 is doing the wrong >>>>>>> thing. >>>>>>> >>>>>>> >>>>>>> >>>>>> I goofed and reversed the two times when writing that. As is always >>>>>> the case with this sort of thing, the unsafe runs are the fast ones. >>>>>> ext3 does not ever do the right thing no matter how you configure it, >>>>>> you have to compensate for its limitations with correct hardware setup >>>>>> to make database writes reliable. >>>>>> >>>>>> >>>>>> >>>>> OK, so if I want the 15 minute speed, I need to give up safety (OK in >>>>> this case as this is just research testing), or see if I can tune >>>>> postgres better. >>>>> >>>>> >>>> Or use a trustworthy hardware caching battery backed RAID controller, >>>> either in RAID mode or JBOD mode. >>>> >>>> >>> Right, because the real danger here is if the power goes out you can end >>> up with a scrambled database, correct? >>> >> Correct. Assuming you can get power applied again before the battery >> in the RAID controller dies, it will then flush out its cache and your >> data will still be coherent. >> > Or if you really don't care if your database is scrambled after a power > outage you can go without the battery backed RAID controller. I do that all the time. On slony replication slaves. You can use a considerably less powerful machine, IO wise, with fsync disabled and a handful of cheap SATA drives. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
