Message-ID: <CAHB1NagzeV+RdndRWU5eXY-NdBuG+mhOccke-fQ0FkVJmQkhFA@mail.gmail.com>
Date: Fri, 13 Jun 2025 10:31:22 +0800
From: Julian Sun <sunjunchao2870@...il.com>
To: "Theodore Ts'o" <tytso@....edu>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>
Subject: Re: Discrepancy between mkfs.ext4 man page and code on
 lazy_journal_init default

On Fri, Jun 13, 2025 at 5:50 AM Theodore Ts'o <tytso@....edu> wrote:
>
> On Thu, Jun 12, 2025 at 08:54:59PM +0800, Julian Sun wrote:
> > Hi,
> >
> > Recently, I observed a significant difference in the number of
> > blk_mq_get_tag() calls when executing mkfs.ext4 -F -q /dev/$nvme
> > versus mkfs.ext4 -F -q -E lazy_itable_init=1,lazy_journal_init=1
> > /dev/$nvme. The former has over 2,000 more calls than the latter,
> > which is confusing because the mkfs.ext4 man page states both features
> > should be enabled by default. This implies the commands should be
> > equivalent, with no I/O difference.
> >
> >        lazy_journal_init[= <0 to disable, 1 to enable>]
> >              If  enabled, the journal inode will not be
> > fully zeroed out by mke2fs.  This speeds up file system initialization
> > noticeably, but carries some small risk if the system crashes before
> > the journal has been overwritten entirely one time.  If the option
> > value is omitted, it defaults to 1 to enable lazy journal inode
> >                               ^^^^^^^^^^^
> > zeroing.
>
> I agree that this might be a bit misleading, but what was meant was
> that:
>
>         mke2fs -E lazy_journal_init
>
> and
>
>         mke2fs -E lazy_journal_init=1
>
> are identical.  The key words here is "If the option value is
> omitted".
>

That's a bit surprising... Because for readers searching for the
default value in the man page, they actually want to know the value
when the configuration is completely unspecified, including in
/etc/mke2fs.conf and when not specified via mke2fs -E. It feels like
the current man page description avoids the majority of scenarios and
only describes a rare case.

What do you think about changing the man page description to: "If the
value is not specified in /etc/mke2fs.conf or via mke2fs, it defaults
to disabled."?
This also implicitly conveys that the default value can be overridden
by configurations in /etc/mke2fs.conf or via the mkfs.ext4 -E option.
>
> Note that there is a distinct difference between the extended option
> using -E command-line option and specifying the default in
> mke2fs.conf.  That is documented in the mke2fs.conf(5) man page.
>
> So the bottom line is that it is possible to change the default of
> lazy_journal_init (and lazy_itable_init, etc.) in /etc/mke2fs.conf.
> So specifying exactly what the default should be is tricky, because
> the system administrator could have changed what is in
> /etc/mke2fs.conf.
>
> So there is the default if there is no mention of the option in
> /etc/mke2fs.conf; the default that is used if the extended option -E
> lazy_itable_init=N is not specified (which is the value in
> /etc/mke2fs.conf, or the default if it is not mentioned in
> /etc/mke2fs.conf); and then there is the default value if "=N" is not
> specified.
>
> 'm not sure what's the best way of making this more explicit, short
> of doubling or tripling the paragraphs in man pages for mke2fs(8) and
> mke2fs.conf(5).  Which would not be ideal....  I'm happy to receive
> any suggestions for how to make things a bit more clear but hopefully
> in a succint way.
>
> Fortunately, it's super rare that users would ever need to change the
> default, and most of the time, it's best not to mess with these knobs
> at all....
>
> Cheers,
>
>                                                 - Ted


Thanks,
-- 
Julian Sun <sunjunchao2870@...il.com>

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