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

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Date: Wed, 24 Apr 2024 14:33:28 +0800
From: Dongsheng Yang <dongsheng.yang@...ystack.cn>
To: 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)



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.

Please correct me if I am wrong.

Thanx
> .
>