Re: PANIC caused by open_sync on Linux

Greg Smith <gsmith(at)gregsmith(dot)com> writes:
> On Fri, 26 Oct 2007, ITAGAKI Takahiro wrote:
>> Mixed usage of buffered and direct i/o is legal, but enforces complexity
>> to kernels. If we simplify it, things would be more relaxed. For
>> example, dropping zero-filling and only use direct i/o. Is it possible?

> It's possible, but performance suffers considerably. I played around with
> this at one point when looking into doing all database writes as sync
> writes. Having to wait until the entire 16MB WAL segment made its way to
> disk before more WAL could be written can cause a nasty pause in activity,
> even with direct I/O sync writes. Even the current buffered zero-filled
> write of that size can be a bit of a drag on performance for the clients
> that get caught behind it, making it any sort of sync write will be far
> worse.

This ties into a loose end we didn't get to yet: being more aggressive
about creating future WAL segments. ISTM there is no good reason for
clients ever to have to wait for WAL segment creation --- the bgwriter,
or possibly the walwriter, ought to handle that in the background. But
we only check for the case once per checkpoint and we don't create a
segment unless there's very little space left.

regards, tom lane