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Subject: Re: [f2fs-dev] Multidevice f2fs mount after disk rearrangement
To:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
References: <4c6cf8418236145f7124ac61eb2908ad@natalenko.name>
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 <20200106183450.GC50058@jaegeuk-macbookpro.roam.corp.google.com>
 <ee2cb1d7a6c1b51e1c8277a8feaafe6d@natalenko.name>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <815fbd14-0fbd-9c44-8d86-4ab13a12dc7f@huawei.com>
Date:   Wed, 8 Jan 2020 16:52:35 +0800
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On 2020/1/7 2:40, Oleksandr Natalenko via Linux-f2fs-devel wrote:
> Hi.
> 
> On 06.01.2020 19:34, Jaegeuk Kim wrote:
>> Thank you for investigating this ahead of me. :) Yes, the device list 
>> is stored
>> in superblock, so hacking it manually should work.
>>
>> Let me think about a tool to tune that.
> 
> Thank you both for the replies.
> 
> IIUC, tune.f2fs is not there yet. I saw a submission, but I do not see 
> it as accepted, right?
> 
> Having this in tune.f2fs would be fine (assuming the assertion is 
> replaced with some meaningful hint message), but wouldn't it be more 
> convenient for an ordinary user to have implemented something like:
> 
> # mount -t f2fs /dev/sdb -o nextdev=/dev/sdc /mnt/fs

Hmm... sounds reasonable, however, the risk is obvious, if we mount with wrong
primary device, filesystem can be aware that with metadata sanity check, if we
mount with wrong secondary/... devices by mistake (or intentionally, people
may think filesystem should be aware illegal parameters....), filesystem won't
be aware of that, then metadata/data will be inconsistent...

Although that may also happen when we use tunesb.f2fs, but fsck.f2fs can be
followed to verify the modification of tunesb.f2fs, that would be much safer.

So I suggest we can do that in tools first, maybe implement nextdev mount option
if we have added metadata in secondary/... device.

Thanks,

> 
> Hm?
>