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Date:   Thu, 1 Sep 2022 11:21:31 +0800
From:   Gao Xiang <hsiangkao@linux.alibaba.com>
To:     Jia Zhu <zhujia.zj@bytedance.com>
Cc:     linux-erofs@lists.ozlabs.org, xiang@kernel.org, chao@kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        yinxin.x@bytedance.com, jefflexu@linux.alibaba.com,
        huyue2@coolpad.com
Subject: Re: [RFC PATCH 0/5] Introduce erofs shared domain
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        linux-erofs@lists.ozlabs.org, xiang@kernel.org, chao@kernel.org,
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        yinxin.x@bytedance.com, jefflexu@linux.alibaba.com,
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Hi Jia,

On Wed, Aug 31, 2022 at 08:31:20PM +0800, Jia Zhu wrote:
> [Kernel Patchset]
> ===============
> Git tree:
> 	https://github.com/userzj/linux.git zhujia/shared-domain-v1
> Git web:
> 	https://github.com/userzj/linux/tree/zhujia/shared-domain-v1
> 
> [Background]
> ============
> In ondemand read mode, we use individual volume to present an erofs
> mountpoint, cookies to present bootstrap and data blobs.
> 
> In which case, since cookies can't be shared between fscache volumes,
> even if the data blobs between different mountpoints are exactly same,
> they can't be shared.
> 
> [Introduction]
> ==============
> Here we introduce erofs shared domain to resolve above mentioned case.
> Several erofs filesystems can belong to one domain, and data blobs can
> be shared among these erofs filesystems of same domain.

As we discussed in the previous community meeting, I agree that is useful
and it's the prerequisite of storage/page cache sharing between blocks
among different images (filesystems).

Thanks for your time and effort on this!

> 
> [Usage]
> Users could specify 'domain_id' mount option to create or join into a
> domain which reuses the same cookies(blobs).
> 
> [Design]
> ========
> 1. Use pseudo mnt to manage domain's lifecycle.
> 2. Use a linked list to maintain & traverse domains.
> 3. Use pseudo sb to create anonymous inode for recording cookie's info
>    and manage cookies lifecycle.
> 
> [Flow Path]
> ===========
> 1. User specify a new 'domain_id' in mount option.
>    1.1 Traverse domain list, compare domain_id with existing domain.[Miss]
>    1.2 Create a new domain(volume), add it to domain list.
>    1.3 Traverse pseudo sb's inode list, compare cookie name with
>        existing cookies.[Miss]
>    1.4 Alloc new anonymous inodes and cookies.
> 
> 2. User specify an existing 'domain_id' in mount option and the data
>    blob is existed in domain.
>    2.1 Traverse domain list, compare domain_id with existing domain.[Hit]
>    2.2 Reuse the domain and increase its refcnt.
>    2.3 Traverse pseudo sb's inode list, compare cookie name with
>    	   existing cookies.[Hit]
>    2.4 Reuse the cookie and increase its refcnt.
> 
> [Test]
> ======
> Git web: (More test cases will be added.)
> 	https://github.com/userzj/demand-read-cachefilesd/tree/shared-domain

I'd suggest integrating to erofs-utils testcases directly, see
https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/log/?h=experimental-tests-fscache 

Thanks,
Gao Xiang