On Thu, 2005-06-02 at 11:49 -0700, Mary Edie Meredith wrote: > My understanding is that O_DIRECT means "direct" as in "no buffering by > the OS" which implies that if you write from your buffer, the write is > not going to return unless the OS thinks the write is completed
Right, I think that's definitely the case. The question is whether a write() under O_DIRECT will also flush the disk's write cache -- i.e. when the write() completes, we need it to be durable over a spontaneous power loss. fsync() or O_SYNC should provide this (modulo braindamaged IDE hardware), but I wouldn't be surprised if O_DIRECT by itself will not (otherwise you would hurt the performance of applications using O_DIRECT that don't need these durability guarantees). > Bottom line: if you do not implement direct/async IO so that you > optimize caching of hot database objects and minimize memory utilization > of objects used once, you are probably leaving performance on the table > for datafiles. Absolutely -- patches are welcome :) I agree async IO + O_DIRECT in some form would be interesting, but the changes required are far from trivial -- my guess is there are lower hanging fruit. -Neil ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
