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From:   David Howells <dhowells@redhat.com>
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To:     Andreas Dilger <adilger@dilger.ca>
Cc:     dhowells@redhat.com, Theodore Ts'o <tytso@mit.edu>,
        "Darrick J. Wong" <djwong@kernel.org>, Chris Mason <clm@fb.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        xfs <linux-xfs@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        linux-cachefs@redhat.com,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        NeilBrown <neilb@suse.com>
Subject: Re: How capacious and well-indexed are ext4, xfs and btrfs directories?
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Date:   Wed, 26 May 2021 01:00:27 +0100
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Andreas Dilger <adilger@dilger.ca> wrote:

> > Any thoughts on how that might scale?  vfs_tmpfile() doesn't appear to
> > require the directory inode lock.  I presume the directory is required for
> > security purposes in addition to being a way to specify the target
> > filesystem.
> 
> I don't see how that would help much?

When it comes to dealing with a file I don't have cached, I can't probe the
cache file to find out whether it has data that I can read until I've opened
it (or found out it doesn't exist).  When it comes to writing to a new cache
file, I can't start writing until the file is created and opened - but this
will potentially hold up close, data sync and writes that conflict (and have
to implicitly sync).  If I can use vfs_tmpfile() to defer synchronous
directory accesses, that could be useful.

As mentioned, creating a link to a temporary cache file (ie. modifying the
directory) could be deferred to a background thread whilst allowing file I/O
to progress to the tmpfile.

David