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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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