## What this teaches

How to performance degradation after upgrading from 8.6.1 to 8.16.1.

## The problem

Issue elastic/elasticsearch#118623 (closed, 21 comments): Performance degradation after upgrading from 8.6.1 to 8.16.1. Investigation pointed at file descriptor leaks on the upgraded nodes.

## What worked (verified answer)

If your cluster slows down after upgrading from 8.6.x to 8.16.x, check for file descriptor pressure before adding hardware. The thread traced the degradation to a file descriptor leak on the upgraded nodes: lots of descriptors held on deleted docvalues .dvd files, a much higher flush rate, and high CPU. Setting sharedArenaMaxPermits=1 resolved it for the reporters, who confirmed it against the same symptoms in a separate discuss thread.

## Source

gh:elastic/elasticsearch#118623 - https://github.com/elastic/elasticsearch/issues/118623