Subject: Re: [RFC PATCH] overlayfs: support more than one read-only layer
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From: Erez Zadok <ezk@fsl.cs.sunysb.edu>
In-Reply-To: <20141110090954.GB333@tucsk>
Date: Mon, 10 Nov 2014 12:08:13 -0500
Cc: Jordi Pujol Palomer <jordipujolp@gmail.com>, linux-unionfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Howells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
        "A. Wan" <jm@mokwan.com>,
        Patrick Frisch <patrick@mekufemo6lbi9soa.myfritz.net>,
        Aaron Campbell <aaron@arbor.net>
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Message-Id: <376686B7-FF3A-4487-A457-EEA5C110B6F8@fsl.cs.sunysb.edu>
References: <20141107170242.GA333@tucsk> <20141108192723.444cecd2@gmail.com> <20141110090954.GB333@tucsk>
To: Miklos Szeredi <miklos@szeredi.hu>
Sender: linux-kernel-owner@vger.kernel.org

Indeed, Miklos, changing layers dynamically is VERY tricky.  Unionfs supports that, but a good amount of its code all over is dedicated to handling dynamic layer changes.  I advise you to avoid supporting this feature for a while, if ever: get a small, functional, working overlayfs first.

I personally never liked the idea of changing layers dynamically: it’s like asking a file system to remain consistent and detect all lower changes when someone goes and hand-edits disk blocks.  Too messy.  The pain of developing and supporting such a feature is much greater than the annoyance to users who have to unmount and remount back their layers to form a new structure.

Cheers,
Erez.

> On Nov 10, 2014, at 4:09 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
> 
> As for changing the stacking while the overlayfs is mounted: currently this is
> not supported, the layers specified at the mount time remain there until the
> overlay is unmounted.  Currently there's no possibility to add or remove layers
> in a dynamic way, and it is definitely more tricky to implement than the static
> configuration.
> 
> Thanks,
> Miklos

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