linux-kernel - Re: Re: dm overlaybd: targets mapping OverlayBD image

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Date:   Fri, 26 May 2023 18:26:33 +0800
From:   Du Rui <durui@...ux.alibaba.com>
To:     alexl@...hat.com
Cc:     agk@...hat.com, dm-devel@...hat.com, durui@...ux.alibaba.com,
        gscrivan@...hat.com, linux-kernel@...r.kernel.org,
        snitzer@...nel.org
Subject: Re: Re: dm overlaybd: targets mapping OverlayBD image

Hi Alexander,

> all the lvm volume changes and mounts during runtime caused
> weird behaviour (especially at scale) that was painful to manage (just
> search the docker issue tracker for devmapper backend). In the end
> everyone moved to a filesystem based implementation (overlayfs based).

Yes, we had exactly the same experience. This is another reason why
this proposal is for dm and lvm, not for container. 
(BTW, we are using TCMU and ublk for overlaybd in production. They are awesome.)

> This solution doesn't even allow page cache sharing between shared
> layers (like current containers do), much less between independent
> layers.

Page cache sharing can be realized with DAX support of the dm targets
(and the inner file system), together with virtual pmem device backend.

> Erofs already has some block-level support for container images

It is interesting. Erofs runs insider a block device in the first place,
like what many file systems do. But do you konw why it implements another 
"some block-level support" by itself?

> And this new approach doesn't help
No. It is intended for dm and lvm.