Re: BUG #16314: Database Cache Hit Ratio (Warning)

On Tue, Mar 24, 2020 at 11:44:10AM -0400, Jeff Janes wrote:
>On Tue, Mar 24, 2020 at 8:06 AM PG Bug reporting form <
>noreply@postgresql.org> wrote:
>
>> The following bug has been logged on the website:
>>
>> Bug reference:      16314
>> Logged by:          Rajiv Ranjan
>> Email address:      rajiv.mca08@gmail.com
>> PostgreSQL version: 9.6.0
>> Operating system:   Red Hat Enterprise Linux Server release 7.6 (Maip)
>> Description:
>>
>> Hi,
>>
>> Currently, we are receiving a warning "Database Cache Hit Ratio
>> (%)(Warning)" from one of the monitoring tools.
>>
>
>This is not a bug in PostgreSQL.  It is arguably a bug in your unnamed
>monitoring tool.
>

Yeah, not a PostgreSQL bug.

>It is a dumb warning. The disk is there to be read from--doing so is not
>inherently a problem. Ignore the warning or disable it.
>

I wouldn't say it's entirely dumb, improving cache hit ratio may make 
huge difference, but it requires a lot more information than was
provided.

Rajiv, please send your question to pgsql-general or pgsql-performance
list, and be sure to include information about the system (amount of RAM
for example), resource utilization, workload, threshold currently used
by the alert, actual cache hit ratio, etc.

>Are the queries you care about too slow because data is occasionally read
>from disk?
>

Yeah, it makes little sense to deal with this unless you can demonstrate
some practical impact. In my experience low cache hit ratio values are
an issue with write-intensive workloads, in which case it may cause
significant write amplification.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services