Re: x206-x225

On Sat, 2006-03-11 at 11:59 -0500, Tom Lane wrote:
> Joost Kraaijeveld <J(dot)Kraaijeveld(at)Askesis(dot)nl> 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(dot)Kraaijeveld(at)Askesis(dot)nl
web: www.askesis.nl