linux-kernel - Re: [GIT PULL v2] timestamp fixes

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Message-ID: <20230922101943.GA13697@twin.jikos.cz>
Date:   Fri, 22 Sep 2023 12:19:43 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>
Subject: Re: [GIT PULL v2] timestamp fixes

On Thu, Sep 21, 2023 at 12:28:13PM -0700, Linus Torvalds wrote:
> On Thu, 21 Sept 2023 at 11:51, Jeff Layton <jlayton@...nel.org> wrote:
> >
> > We have many, many inodes though, and 12 bytes per adds up!
> 
> That was my thinking, but honestly, who knows what other alignment
> issues might eat up some - or all - of the theoreteical 12 bytes.
> 
> It might be, for example, that the inode is already some aligned size,
> and that the allocation alignment means that the size wouldn't
> *really* shrink at all.
> 
> So I just want to make clear that I think the 12 bytes isn't
> necessarily there. Maybe you'd get it, maybe it would be hidden by
> other things.

I think all filesystem developers appreciate when struct inode shrinks,
it's usually embedded with additional data and the size grows. I'm on a
mission to squeeze btrfs_inode under 1024 so it fits better to the slab
pages and currently it's about 1100 bytes. 1024 is within reach but it
gets harder to find potential space savings.