On Fri, Sep 27, 2024 at 9:11 AM veem v <veema0000@xxxxxxxxx> wrote:
On Thu, 26 Sept 2024 at 16:33, yudhi s <learnerdatabase99@xxxxxxxxx> wrote:Hello All,
In a RDS postgres we are seeing some select queries when running and doing sorting on 50 million rows(as its having order by clause in it) , the significant portion of wait event is showing as "IO:BufFileWrite" and it runs for ~20minutes+.
Going through the document in the link below, it states we should monitor the "FreeLocalStorage" metric and when monitoring that, I see it showing up to ~535GB as the max limit and when these queries run this goes down till 100GB. Note- (it's a R7g8xl instance)
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/apg-waits.iobuffile.html
We were thinking of bumping up the work_mem to a higher value in database level , which is currently having size 4MB default. But we will also have ~100 sessions running at same time and majority were from other applications which execute other single row "insert" queries and I hope that will not need high "work_mem" . And setting it at database level will consume 100 times that set work_mem value. So how to handle this situation?
Or
Is it fine to let it use "FreeLocalStorage" unless it goes till zero?
Also I am confused between the local storage (which is showing as 535GB) vs the memory/RAM which is 256GB for this instance class with ~128TB max storage space restriction, how are these storage different, (mainly the 535GB space which it's showing vs the 128TB storage space restriction)? Appreciate your guidance.
select query looks something as below with no Joins but just single table fetch:-
Select....
from <table_name>
where
order by column1, column2 LIMIT $b1 OFFSET $B2 ;My 2 centsI think you should set the work_mem on specific session level , if your sorting queries are only from specific handful of sessions, as because setting it up at database level will eat up your most of RAM(which you said is 256GB) and you said 100+ sessions getting spawned at any point in time.
Thank you.
When I checked pg_stat_statements for this query , and divided the temp_blk_read+temp_blk_written with the "calls", it came as ~1million which means ~7GB. So does that mean ~7GB of work_mem should be allocated for this query?
