linux-kernel - Re: [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device)

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Date: Wed, 24 Apr 2024 11:08:26 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Dongsheng Yang <dongsheng.yang@...ystack.cn>, Dan Williams
	<dan.j.williams@...el.com>, <axboe@...nel.dk>, John Groves <John@...ves.net>
CC: <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, Dongsheng Yang <dongsheng.yang.linux@...il.com>
Subject: Re: [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device)

Dongsheng Yang wrote:
> 
> 
> 在 2024/4/24 星期三 下午 12:29, Dan Williams 写道:
> > Dongsheng Yang wrote:
> >> From: Dongsheng Yang <dongsheng.yang.linux@...il.com>
> >>
> >> Hi all,
> >> 	This patchset introduce cbd (CXL block device). It's based on linux 6.8, and available at:
> >> 	https://github.com/DataTravelGuide/linux
> >>
> > [..]
> >> (4) dax is not supported yet:
> >> 	same with famfs, dax device is not supported here, because dax device does not support
> >> dev_dax_iomap so far. Once dev_dax_iomap is supported, CBD can easily support DAX mode.
> > 
> > I am glad that famfs is mentioned here, it demonstrates you know about
> > it. However, unfortunately this cover letter does not offer any analysis
> > of *why* the Linux project should consider this additional approach to
> > the inter-host shared-memory enabling problem.
> > 
> > To be clear I am neutral at best on some of the initiatives around CXL
> > memory sharing vs pooling, but famfs at least jettisons block-devices
> > and gets closer to a purpose-built memory semantic.
> > 
> > So my primary question is why would Linux need both famfs and cbd? I am
> > sure famfs would love feedback and help vs developing competing efforts.
> 
> Hi,
> 	Thanks for your reply, IIUC about FAMfs, the data in famfs is stored in 
> shared memory, and related nodes can share the data inside this file 
> system; whereas cbd does not store data in shared memory, it uses shared 
> memory as a channel for data transmission, and the actual data is stored 
> in the backend block device of remote nodes. In cbd, shared memory works 
> more like network to connect different hosts.
> 
> That is to say, in my view, FAMfs and cbd do not conflict at all; they 
> meet different scenario requirements. cbd simply uses shared memory to 
> transmit data, shared memory plays the role of a data transmission 
> channel, while in FAMfs, shared memory serves as a data store role.

If shared memory is just a communication transport then a block-device
abstraction does not seem a proper fit. From the above description this
sounds similar to what CONFIG_NTB_TRANSPORT offers which is a way for
two hosts to communicate over a shared memory channel.

So, I am not really looking for an analysis of famfs vs CBD I am looking
for CBD to clarify why Linux should consider it, and why the
architecture is fit for purpose.