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Subject: Re: [RFC] Thing 1: Shardmap for Ext4
To:     Dave Chinner <david@fromorbit.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>
Cc:     Andreas Dilger <adilger@dilger.ca>, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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 <20191204234106.GC5641@mit.edu> <20191206011640.GQ2695@dread.disaster.area>
From:   Daniel Phillips <daniel@phunq.net>
Message-ID: <1dd1f9f6-89a4-e73a-d7b9-94a12412876c@phunq.net>
Date:   Thu, 5 Dec 2019 21:09:28 -0800
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On 2019-12-05 5:16 p.m., Dave Chinner wrote:
> On Wed, Dec 04, 2019 at 06:41:06PM -0500, Theodore Y. Ts'o wrote:
>> On Wed, Dec 04, 2019 at 11:31:50AM -0700, Andreas Dilger wrote:
>>> One important use case that we have for Lustre that is not yet in the
>>> upstream ext4[*] is the ability to do parallel directory operations.
>>> This means we can create, lookup, and/or unlink entries in the same
>>> directory concurrently, to increase parallelism for large directories.
>>>
>>> [*] we've tried to submit the pdirops patch a couple of times, but the
>>> main blocker is that the VFS has a single directory mutex and couldn't
>>> use the added functionality without significant VFS changes.
>>> Patch at https://git.whamcloud.com/?p=fs/lustre-release.git;f=ldiskfs/kernel_patches/patches/rhel8/ext4-pdirop.patch;hb=HEAD
>>>
>>
>> The XFS folks recently added support for parallel directory operations
>> into the VFS, for the benefit of XFS has this feature.
> 
> The use of shared i_rwsem locking on the directory inode during
> lookup/pathwalk allows for concurrent lookup/readdir operations on
> a single directory. However, the parent dir i_rwsem is still held
> exclusive for directory modifications like create, unlink, etc.
> 
> IOWs, the VFS doesn't allow for concurrent directory modification
> right now, and that's going to be the limiting factor no matter what
> you do with internal filesystem locking.

On a scale of 0 to 10, how hard do you think that would be to relax
in VFS, given the restriction of no concurrent inter-directory moves?