On Sat, 2006-03-11 at 11:59 -0500, Tom Lane wrote: > Joost Kraaijeveld <J.Kraaijeveld@xxxxxxxxxx> writes: > > I assume that for PostgreSQL "written to disk" is after fsync returned > > successfully. In practice that could very well mean that the data is > > still in a cache somewhere (controller or harddisk, not in the OS > > anymore, see also man page of fsync) > > What it had better mean, if you want your database to be reliable, > is that the data is stored someplace that will survive a system crash > (power outage, kernel panic, etc). A battery-backed RAM cache is OK, > assuming that total failure of the RAID controller is not one of the > events you consider likely enough to need protection against. Maybe I should have expressed myself better. The parent post said: > It depends if you consider that "written to the disk" means "data is > somewhere between the OS cache and the platter" or "data is written on > the platter and will survive a power loss". > > Postgres wants the second option, of course. With my remark I meant that the only thing *PostgreSQL* can expect is that the data is out of the OS: there is no greater guarantee in the fsync function. If the *database administrator* wants better guarantees, he (or she) better read your advise. > The description of your SATA drive makes it sound like the drive > does not put data on the platter before reporting "write complete", > but only stores it in on-board RAM cache. It is highly unlikely > that there is any battery backing for that cache, and therefore that > drive is not to be trusted. Yep, the drives have a write cache, and indeed, they are not backed up by a battery (neither is my RAID controller) but as this is a test/development machine, I don't really care. You made me rethink my production machine thought. I will have to check the drives and the state of their write cache of that machine. Thanks for that. -- Groeten, Joost Kraaijeveld Askesis B.V. Molukkenstraat 14 6524NB Nijmegen tel: 024-3888063 / 06-51855277 fax: 024-3608416 e-mail: J.Kraaijeveld@xxxxxxxxxx web: www.askesis.nl
