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From: Dongsheng Yang <dongsheng.yang@easystack.cn>
To: Gregory Price <gregory.price@memverge.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	John Groves <John@groves.net>,
	axboe@kernel.dk, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org,
	nvdimm@lists.linux.dev
Subject: Re: [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device)
Date: Mon, 3 Jun 2024 09:33:29 +0800	[thread overview]
Message-ID: <a04a5bbc-a44b-57e4-0fa6-0ce84b18a395@easystack.cn> (raw)
In-Reply-To: <Zlndc8NI0eK3MmuR@memverge.com>



在 2024/5/31 星期五 下午 10:23, Gregory Price 写道:
> On Thu, May 30, 2024 at 02:59:38PM +0800, Dongsheng Yang wrote:
>>
>>
>> 在 2024/5/29 星期三 下午 11:25, Gregory Price 写道:
>>>
>>> There are some (FAMFS, for example). The coherence state of these
>>> systems tend to be less volatile (e.g. mappings are read-only), or
>>> they have inherent design limitations (cacheline-sized message passing
>>> via write-ahead logging only).
>>
>> Can you explain more about this? I understand that if the reader in the
>> writer-reader model is using a readonly mapping, the interaction will be
>> much simpler. However, after the writer writes data, if we don't have a
>> mechanism to flush and invalidate puncturing all caches, how can the
>> readonly reader access the new data?
> 
> This is exactly right, so the coherence/correctness of the data needs to
> be enforced in some other way.
> 
> Generally speaking, the WPQs will *eventually* get flushed.  As such,
> the memory will *eventually* become coherent.  So if you set up the
> following pattern, you will end up with an "eventually coherent" system


Yes, it is "eventually coherent" if "NO CLEAN WRITEBACK" bit in both 
CSDS and DVSEC is set.
> 
> 1) Writer instantiates the memory to be used
> 2) Writer calculates and records a checksum of that data into memory
> 3) Writer invalidates everything
> 4) Reader maps the memory
> 5) Reader reads the checksum and calculates the checksum of the data
>     a) if the checksums match, the data is coherent
>     b) if they don't, we must wait longer for the queues to flush

Yes, the checksum was mentioned by John, it is used in FAMFS/pcq_lib.c, 
pcq use sequence and checksum in consumer to make sure data consistency.

I think it's a good idea and was planning to introduce it into cbd, of 
coures it should be optional for cbd, as cbd current only supports
hardware-consistency usage. it can be an option to do data verification.

Thanx
> 
> This is just one example of a system design which enforces coherence by
> placing the limitation on the system that the data will never change
> once it becomes coherent.
> 
> Whatever the case, regardless of the scheme you come up with, you will
> end up with a system where the data must be inspected and validated
> before it can be used.  This has the limiting factor of performance:
> throughput will be limited by how fast you can validate the data.
> 
> ~Gregory
> .
> 

  reply	other threads:[~2024-06-03  1:59 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-22  7:15 [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device) Dongsheng Yang
2024-04-22  7:16 ` [PATCH 1/7] block: Init for CBD(CXL " Dongsheng Yang
2024-04-22 18:39   ` Randy Dunlap
2024-04-22 22:41     ` Dongsheng Yang
2024-04-24  3:58   ` Chaitanya Kulkarni
2024-04-24  8:36     ` Dongsheng Yang
2024-04-22  7:16 ` [PATCH 2/7] cbd: introduce cbd_transport Dongsheng Yang
2024-04-24  4:08   ` Chaitanya Kulkarni
2024-04-24  8:43     ` Dongsheng Yang
2024-04-22  7:16 ` [PATCH 3/7] cbd: introduce cbd_channel Dongsheng Yang
2024-04-22  7:16 ` [PATCH 4/7] cbd: introduce cbd_host Dongsheng Yang
2024-04-25  5:51   ` [EXTERNAL] " Bharat Bhushan
2024-04-22  7:16 ` [PATCH 5/7] cbd: introuce cbd_backend Dongsheng Yang
2024-04-24  5:03   ` Chaitanya Kulkarni
2024-04-24  8:36     ` Dongsheng Yang
2024-04-25  5:46   ` [EXTERNAL] " Bharat Bhushan
2024-04-22  7:16 ` [PATCH 7/7] cbd: add related sysfs files in transport register Dongsheng Yang
2024-04-25  5:24   ` [EXTERNAL] " Bharat Bhushan
2024-04-22 22:42 ` [PATCH 6/7] cbd: introduce cbd_blkdev Dongsheng Yang
2024-04-23  7:27   ` Dongsheng Yang
2024-04-24  4:29 ` [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device) Dan Williams
2024-04-24  6:33   ` Dongsheng Yang
2024-04-24 15:14     ` Gregory Price
2024-04-26  1:25       ` Dongsheng Yang
2024-04-26 13:48         ` Gregory Price
2024-04-26 14:53           ` Dongsheng Yang
2024-04-26 16:14             ` Gregory Price
2024-04-28  5:47               ` Dongsheng Yang
2024-04-28 16:44                 ` Gregory Price
2024-04-28 16:55                 ` John Groves
2024-05-03  9:52                   ` Jonathan Cameron
2024-05-08 11:39                     ` Dongsheng Yang
2024-05-08 12:11                       ` Jonathan Cameron
2024-05-08 13:03                         ` Dongsheng Yang
2024-05-08 15:44                           ` Jonathan Cameron
2024-05-09 11:24                             ` Dongsheng Yang
2024-05-09 12:21                               ` Jonathan Cameron
2024-05-09 13:03                                 ` Dongsheng Yang
2024-05-21 18:41                                   ` Dan Williams
2024-05-22  6:17                                     ` Dongsheng Yang
2024-05-29 15:25                                       ` Gregory Price
2024-05-30  6:59                                         ` Dongsheng Yang
2024-05-30 13:38                                           ` Jonathan Cameron
2024-06-01  3:22                                             ` Dan Williams
2024-06-03 12:48                                               ` Jonathan Cameron
2024-06-03 17:28                                                 ` James Morse
2024-06-04 14:26                                                   ` Jonathan Cameron
2024-05-31 14:23                                           ` Gregory Price
2024-06-03  1:33                                             ` Dongsheng Yang [this message]
2024-04-30  0:34                 ` Dan Williams
2024-04-24 18:08     ` Dan Williams
     [not found]       ` <539c1323-68f9-d753-a102-692b69049c20@easystack.cn>
2024-04-30  0:10         ` Dan Williams

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