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Date:   Wed, 11 May 2022 12:33:33 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Shaun Tancheff <shaun.tancheff@....com>
Cc:     shaun@...cheff.com, Andreas Dilger <adilger.kernel@...ger.ca>,
        "open list:EXT4 FILE SYSTEM" <linux-ext4@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Shrink fast commit buffer when not used

On Thu, Apr 07, 2022 at 07:42:44PM +0700, Shaun Tancheff wrote:
> Shrink the fast-commit buffer when the feature is not
> enabled. By default the fast-commit buffer will allocate 256
> blocks if s_num_fc_blks is 0. Set s_num_fc_blks to a smaller
> value (> 0) to avoid allocating a large unused buffer, this
> also makes more journal credits available when fast commit
> is not used.
> 
> Signed-off-by: Shaun Tancheff <shaun.tancheff@....com>

The journal->j_superblock data structure is stored on disk, so when
you make this change, it's can and will get written back to disk, at
which point the s_num_fc_blks is permanently strunk.  If the file
system might be mounted with the mount option data=journal mode,
fast_commit will be disabled; but it might be subsequently mounted
without this mount option.

Why do you believe this patch is necessary?  If there is a file system
which is only going to be mounted using data=journal, the file system
should simply not be formwatted with the fast_commit option.

       	      	     		     	 - Ted

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