Re: Fix parallel vacuum buffer usage reporting

Hi,

Thank you for working on this!

On Wed, 1 May 2024 at 06:37, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> Thank you for further testing! I've pushed the patch.

I realized a behaviour change while looking at 'Use pgBufferUsage for
block reporting in analyze' thread [1]. Since that change applies here
as well, I thought it is better to mention it here.

Before this commit, VacuumPageMiss did not count the blocks if its
read was already completed by other backends [2]. Now,
'pgBufferUsage.local_blks_read + pgBufferUsage.shared_blks_read'
counts every block attempted to be read; possibly double counting if
someone else has already completed the read. I do not know which
behaviour is correct but I wanted to mention this.

[1] https://postgr.es/m/CAO6_Xqr__kTTCLkftqS0qSCm-J7_xbRG3Ge2rWhucxQJMJhcRA%40mail.gmail.com

[2] In the WaitReadBuffers() function, see comment:
/*
* Skip this block if someone else has already completed it. If an
* I/O is already in progress in another backend, this will wait for
* the outcome: either done, or something went wrong and we will
* retry.
*/

--
Regards,
Nazir Bilal Yavuz
Microsoft