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. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
