chore(docs): Update Performance documentation to include SDio Flag

[Shubhang]

Overview

Setting the +A flag currently doesn't do anything meaningful to the performance of CouchDB.

Testing recommendations

Doc update

Related Issues or Pull Requests

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches

[nickva]

Great idea @Shubhang to update the docs. Thank you for your contribution.

I had a few corrections in a comment (the flag name) and a suggestion to improve it even better by removing the whole +A section and ERL_FLAGS and replace it with more current advice. What do you think?

[nickva]

+Sdio 80"

Note, the flag is +SDio ... (capital D) (double-check at https://www.erlang.org/doc/apps/erts/erl_cmd.html#emulator-flags)

I wonder since the +A advice doesn't apply any longer maybe we should mention the correct way of updating the value and to use vm.args https://github.com/apache/couchdb/blob/main/rel/overlay/etc/vm.args#L71-L73. Maybe at the end, as minor note, we could add that this used to be an +A and that it doesn't apply any longer, and that +A setting should be removed.

(suggestion)

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...However, there is one option that can increase the responsiveness of your CouchDB server when disk performance is a bottleneck, and that is setting the number dirty IO schedulers for the Erlang VM.

The setting is +SDio N where N is the number of dirty IO schedulers. CouchDB releases version 3.3.0 and higher set the value in the vm.args file as +SDio 16. Larger machines with more CPUs and faster disks could use a larger value like +SDio 80, for example.

These schedulers can also be configured by ERL_FLAGS environment variable. For example, to give Erlang 80 dirty schedulers add the following to (prefix)/etc/defaults/couchdb (or equivalent)::

export ERL_FLAGS="+SDio 80"
...

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That's just a suggestion, it can probably use better wording