linux-kernel - RE: [PATCH for-next v5 0/7] On-Demand Paging on SoftRoCE

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Message-ID: <TYCPR01MB8455648A5C9DFFB37FFB172FE57C9@TYCPR01MB8455.jpnprd01.prod.outlook.com>
Date:   Fri, 19 May 2023 09:57:03 +0000
From:   "Daisuke Matsuda (Fujitsu)" <matsuda-daisuke@...itsu.com>
To:     'Guoqing Jiang' <guoqing.jiang@...ux.dev>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "leonro@...dia.com" <leonro@...dia.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "zyjzyj2000@...il.com" <zyjzyj2000@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rpearsonhpe@...il.com" <rpearsonhpe@...il.com>,
        "Xiao Yang (Fujitsu)" <yangx.jy@...itsu.com>,
        "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>,
        "Yasunori Gotou (Fujitsu)" <y-goto@...itsu.com>
Subject: RE: [PATCH for-next v5 0/7] On-Demand Paging on SoftRoCE

On Fri, May 19, 2023 3:42 PM Guoqing Jiang wrote:
> 
> Hello,
> 
> On 5/18/23 16:21, Daisuke Matsuda wrote:
> > [2] [PATCH for-next v3 0/7] On-Demand Paging on SoftRoCE
> > https://lore.kernel.org/lkml/cover.1671772917.git.matsuda-daisuke@fujitsu.com/
> 
> Quote from above link
> 
> "There is a problem that data on DAX-enabled filesystem cannot be
> duplicated with
> software RAID or other hardware methods."
> 
> Could you elaborate a bit more about the problem or any links about it?
> Thank you.

I am not an expert of Pmems, but my understanding is as follows:

Pmem (Persistent memory) is detected as memory device during boot process.
Physical addresses are allocated to them just like other memory in DIMM slots,
so system have to treat them differently from traditional storage devices like HDD/SSD.

It may be technically possible to duplicate data using multiple Pmems, but the duplication
is practically not useful. For traditional storage devices, you can hot-remove and hot-add
them easily on failure. However, Pmems are not attached to hot-pluggable slots. You have
to halt the system and open the cabinet to change out the Pmem. This means availability
of the system is not improved with data duplication on the same host.

Daisuke

> 
> Guoqing