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chore(docs): Update Performance documentation to include SDio Flag #5870

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Update performance.rst
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8 changes: 8 additions & 0 deletions src/docs/src/maintenance/performance.rst
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Expand Up @@ -58,6 +58,14 @@ operations add the following to ``(prefix)/etc/defaults/couchdb``

.. _command line flag +A in erl(1): http://erlang.org/doc/man/erl.html

Since Erlang OTP 21, the linked-in drivers have been migrated to dirty IO schedulers.
These schedulers can 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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@nickva nickva Jan 30, 2026

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+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)

...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"
...

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

System Resource Limits
======================

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