Re: index prefetching

On 1/16/24 2:10 PM, Konstantin Knizhnik wrote:
> Amazon RDS is just vanilla Postgres with file system mounted on EBS 
> (Amazon  distributed file system).
> EBS provides good throughput but larger latencies comparing with local SSDs.
> I am not sure if read-ahead works for EBS.

Actually, EBS only provides a block device - it's definitely not a 
filesystem itself (*EFS* is a filesystem - but it's also significantly 
different than EBS). So as long as readahead is happening somewheer 
above the block device I would expect it to JustWork on EBS.

Of course, Aurora Postgres (like Neon) is completely different. If you 
look at page 53 of [1] you'll note that there's two different terms 
used: prefetch and batch. I'm not sure how much practical difference 
there is, but batched IO (one IO request to Aurora Storage for many 
blocks) predates index prefetch; VACUUM in APG has used batched IO for a 
very long time (it also *only* reads blocks that aren't marked all 
visble/frozen; none of the "only skip if skipping at least 32 blocks" 
logic is used).

1: 

https://d1.awsstatic.com/events/reinvent/2019/REPEAT_1_Deep_dive_on_Amazon_Aurora_with_PostgreSQL_compatibility_DAT328-R1.pdf
-- 
Jim Nasby, Data Architect, Austin TX