Re: Do you know the reason for increased max latency due to xlog scaling?

From: "Jeff Janes" <jeff.janes@gmail.com>
> I thought that this was the point I was making, not the point I was
> missing.  You have the same hard drives you had before, but now due to a
> software improvement you are cramming 5 times more stuff through them.
> Yeah, you will get bigger latency spikes.  Why wouldn't you?  You are now
> beating the snot out of your hard drives, whereas before you were not.
>
> If you need 10,000 TPS, then you need to upgrade to 9.4.  If you need it
> with low maximum latency as well, then you probably need to get better IO
> hardware as well (maybe not--maybe more tuning could help).  With 9.3 you
> didn't need better IO hardware, because you weren't capable of maxing out
> what you already had.  With 9.4 you can max it out, and this is a good
> thing.
>
> If you need 10,000 TPS but only 2000 TPS are completing under 9.3, then
> what is happening to the other 8000 TPS? Whatever is happening to them, it
> must be worse than a latency spike.
>
> On the other hand, if you don't need 10,000 TPS, than measuring max 
> latency
> at 10,000 TPS is the wrong thing to measure.

Thank you, I've probably got the point --- you mean the hard disk for WAL is 
the bottleneck.  But I still wonder a bit why the latency spike became so 
bigger even with # of clients fewer than # of CPU cores.  I suppose the 
requests get processed more smoothly when the number of simultaneous 
requests is small.  Anyway, I want to believe the latency spike would become 
significantly smaller on an SSD.

Regards
MauMau