linux-ext4 - journal_async_commit and external journal

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Message-ID: <36a737ff-1e84-5863-2512-8c85975868c4@nh2.me>
Date:   Sat, 5 May 2018 05:04:53 +0200
From:   Niklas Hambüchen <mail@....me>
To:     linux-ext4@...r.kernel.org
Subject: journal_async_commit and external journal

Hello,

The article https://raid6.com.au/posts/fs_ext4_external_journal_caveats/
is an often-cited source about setting up external journals.

It claims that without the `journal_async_commit` mount option ext4 with
external journal will eventually go read-only.

> Apparently external journal can't be updated in synchronous manner so
sooner or later ext4 switches to read-only mode following journal commit
error.

https://www.arm-blog.com/ext4-journal-nvram-card/ makes the same claim.

> The “journal_async_commit” is mandatory in order to use the external
journal. If you forget to mount the filesystem with this flag it
eventually become read-only and will need a diskcheck before you can
mount it again! This problem occurs because the external journal cannot
sync in synchronised mode.

But the only upstream bugs I could find are records of situations where
_with_ the option ext4 went read-only at boot or remount.

Is the issue described in these articles a known problem?

Thanks!
Niklas