Re: effective_io_concurrency on EBS/gp2

On Wed, Jan 31, 2018 at 1:57 PM, Vitaliy Garnashevich
<vgarnashevich@gmail.com> wrote:
> More tests:
>
> io1, 100 GB:
>
> effective_io_concurrency=0
>  Execution time: 40333.626 ms
> effective_io_concurrency=1
>  Execution time: 163840.500 ms

In my experience playing with prefetch, e_i_c>0 interferes with kernel
read-ahead. What you've got there would make sense if what postgres
thinks will be random I/O ends up being sequential. With e_i_c=0, the
kernel will optimize the hell out of it, because it's a predictable
pattern. But with e_i_c=1, the kernel's optimization gets disabled but
postgres isn't reading much ahead, so you get the worst possible case.