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

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Date:   Tue, 25 May 2021 23:31:50 +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:

> As described elsewhere in the thread, allowing concurrent create and unlink
> in a directory (rename probably not needed) would be invaluable for scaling
> multi-threaded workloads.  Neil Brown posted a prototype patch to add this
> to the VFS for NFS:

Actually, one thing I'm looking at is using vfs_tmpfile() to create a new file
(or a replacement file when invalidation is required) and then using
vfs_link() to attach directory entries in the background (possibly using
vfs_link() with AT_LINK_REPLACE[1] instead of unlink+link).

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.

David

[1] https://lore.kernel.org/linux-fsdevel/cover.1580251857.git.osandov@fb.com/