Re: Experimental patch for inter-page delay in VACUUM

Tom Lane wrote:

> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
>> What still needs to be addressed is the IO storm cause by checkpoints. I
>> see it much relaxed when stretching out the BufferSync() over most of
>> the time until the next one should occur. But the kernel sync at it's
>> end still pushes the system hard against the wall.
>
> I have never been happy with the fact that we use sync(2) at all. Quite
> aside from the "I/O storm" issue, sync() is really an unsafe way to do a
> checkpoint, because there is no way to be certain when it is done. And
> on top of that, it does too much, because it forces syncing of files
> unrelated to Postgres.

Sure does it do too much. But together with the other layer of
indirection, the virtual file descriptor pool, what is the exact
guaranteed behaviour of

write(); close(); open(); fsync();

cross platform?

> Actually, once you build it this way, you could make all writes
> synchronous (open the files O_SYNC) so that there is never any need for
> explicit fsync at checkpoint time. The background writer process would
> be the one incurring the wait in most cases, and that's just fine. In
> this way you could directly control the rate at which writes are issued,
> and there's no I/O storm at all. (fsync could still cause an I/O storm
> if there's lots of pending writes in a single file.)

Yes, but then the configuration leans more towards "take over the RAM"
again, and we better have a much improved cache strategy before that.

Jan

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