Re: posix_fadvise() and pg_receivexlog

On Thu, Aug 7, 2014 at 3:59 AM, Heikki Linnakangas
<hlinnakangas(at)vmware(dot)com> wrote:
> On 08/06/2014 08:39 PM, Fujii Masao wrote:
>>
>> Hi,
>>
>> The WAL files that pg_receivexlog writes will not be re-read soon
>> basically,
>> so we can advise the OS to release any cached pages when WAL file is
>> closed. I feel inclined to change pg_receivexlog that way. Thought?
>
>
> -1. The OS should be smart enough to not thrash the cache by files that are
> written sequentially and never read.

Yep, the OS should be so smart, but I'm not sure if it actually is. Maybe not,
so I was thinking that posix_fadvise is called when the server closes WAL file.

> If we go down this path, we'd need to
> sprinkle posix_fadvises into many, many places.

Yes, that's valid concern. But if we can prove that adding posix_fadvise to
a certain place can improve the performance well, I'm inclined to do that.

> Anyway, who are we to say that they won't be re-read soon? You might e.g
> have a secondary backup site where you copy the files received by
> pg_receivexlog, as soon as they're completed.

So whether posix_fadvise is called or not needs to be exposed as an
user-configurable option. We would need to measure how useful exposing
that is, though.

Regards,

--
Fujii Masao