Re: Performance tuning on FreeBSD

Greg Smith wrote:
> On Mon, 3 Mar 2008, alan bryan wrote:
>
>>> pgbench -c 100 -t 1000 testdb
>> tps = 558.013714 (excluding connections establishing)
>>
>> Just for testing, I tried turning off fsync and got:
>> tps = 4061.662041 (excluding connections establishing)
>
> This is odd. ~500 is what I expect from this test when there is no
> write cache to accelerate fsync, while ~4000 is normal for your class of
> hardware when you have such a cache.

I'm curious about the math behind this - is ~4000 burst or sustained
rate? For common BBU cache sizes (256M, 512M), filling that amount with
data is pretty trivial. When the cache is full, new data can enter the
cache only at a rate at which old data is evacuated from the cache (to
the drive), which is at "normal", uncached disk drive speeds.