man: add note about journald forwarding being sync

This footgun should at least be documented, if there's not going
to be a shortcut setting to establish the async `journalctl
--follow` equivalent.

Fixes: https://github.com/systemd/systemd/issues/2815

diff --git a/man/journald.conf.xml b/man/journald.conf.xml
index 1c0487a4fc6d205009463a5c0c096e4d1557cf18..e150d04dcff24298d3e2c733675b2ce8f7a9c41c 100644 (file)
--- a/man/journald.conf.xml
+++ b/man/journald.conf.xml
@@ -386,6 +386,15 @@
         the log buffer, for example by adding <literal>log_buf_len=8M</literal> to the kernel command line.
         <command>systemd</command> will automatically disable kernel's rate-limiting applied to userspace
         processes (equivalent to setting <literal>printk.devkmsg=on</literal>).</para></listitem>
+
+        <para>Note: Forwarding is performed synchronously within journald, and may significantly affect its
+        performance.  This is particularly relevant when using ForwardToConsole=yes in cloud environments,
+        where the console is often a slow, virtual serial port.  Since journald is implemented as a
+        conventional single-process daemon, forwarding to a completely hung console will block journald.
+        This can have a cascading effect resulting in any services synchronously logging to the blocked
+        journal also becoming blocked.  Unless actively debugging/developing something, it's generally
+        preferable to setup a <command>journalctl --follow</command> style service redirected to the
+        console, instead of ForwardToConsole=yes, for production use.</para>
       </varlistentry>
 
       <varlistentry>