linux-ext4 - Re: How capacious and well-indexed are ext4, xfs and btrfs directories?

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Message-ID: <4176733.1621987227@warthog.procyon.org.uk>
Date:   Wed, 26 May 2021 01:00:27 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     dhowells@...hat.com, Theodore Ts'o <tytso@....edu>,
        "Darrick J. Wong" <djwong@...nel.org>, Chris Mason <clm@...com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        xfs <linux-xfs@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        linux-cachefs@...hat.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        NeilBrown <neilb@...e.com>
Subject: Re: How capacious and well-indexed are ext4, xfs and btrfs directories?

Andreas Dilger <adilger@...ger.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