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Subject: Re: How capacious and well-indexed are ext4, xfs and btrfs
 directories?
To:     Dave Chinner <david@fromorbit.com>,
        David Howells <dhowells@redhat.com>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        "Darrick J. Wong" <djwong@kernel.org>, Chris Mason <clm@fb.com>,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-btrfs@vger.kernel.org, linux-cachefs@redhat.com,
        linux-fsdevel@vger.kernel.org
References: <206078.1621264018@warthog.procyon.org.uk>
 <20210517232237.GE2893@dread.disaster.area>
From:   Avi Kivity <avi@scylladb.com>
Organization: ScyllaDB
Message-ID: <ad2e8757-41ce-41e3-a22e-0cf9e356e656@scylladb.com>
Date:   Wed, 19 May 2021 11:00:03 +0300
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On 18/05/2021 02.22, Dave Chinner wrote:
>
>> What I'd like to do is remove the fanout directories, so that for each logical
>> "volume"[*] I have a single directory with all the files in it.  But that
>> means sticking massive amounts of entries into a single directory and hoping
>> it (a) isn't too slow and (b) doesn't hit the capacity limit.
> Note that if you use a single directory, you are effectively single
> threading modifications to your file index. You still need to use
> fanout directories if you want concurrency during modification for
> the cachefiles index, but that's a different design criteria
> compared to directory capacity and modification/lookup scalability.


Something that hit us with single-large-directory and XFS is that XFS 
will allocate all files in a directory using the same allocation group. 
If your entire filesystem is just for that one directory, then that 
allocation group will be contended. We saw spurious ENOSPC when that 
happened, though that may have related to bad O_DIRECT management by us.


We ended up creating files in a temporary directory and moving them to 
the main directory, since for us the directory layout was mandated by 
compatibility concerns.


We are now happy with XFS large-directory management, but are nowhere 
close to a million files.