linux-kernel - Re: [PATCH v3] exfat: remove EXFAT_SB

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Date:   Tue, 18 Aug 2020 10:20:14 +0900
From:   Tetsuhiro Kohada <kohada.t2@...il.com>
To:     Namjae Jeon <namjae.jeon@...sung.com>
Cc:     kohada.tetsuhiro@...mitsubishielectric.co.jp,
        mori.takahiro@...mitsubishielectric.co.jp,
        motai.hirotaka@...mitsubishielectric.co.jp,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        'Sungjong Seo' <sj1557.seo@...sung.com>
Subject: Re: [PATCH v3] exfat: remove EXFAT_SB_DIRTY flag

Thank you for your reply.

>>> Most of the NAND flash devices and HDDs have wear leveling and bad sector replacement algorithms
>> applied.
>>> So I think that the life of the boot sector will not be exhausted first.
>>
>> I'm not too worried about the life of the boot-sector.
>> I'm worried about write failures caused by external factors.
>> (power failure/system down/vibration/etc. during writing) They rarely occur on SD cards, but occur on
>> many HDDs, some SSDs and USB storages by 0.1% or more.
> Hard disk and SSD do not guarantee atomic write of a sector unit?

In the case of SD, the sector-data will be either new or old when unexpected write interruption occurs.
Almost HDD, the sector-data will be either new, old, or unreadable.
And, some SSD products have similar problem.

>> Especially with AFT-HDD, not only boot-sector but also the following multiple sectors become
>> unreadable.
> Other file systems will also be unstable on a such HW.

A well-designed FileSystems never rewrite critical regions.

>> It is not possible to completely solve this problem, as long as writing to the boot-sector.
>> (I think it's a exFAT's specification defect) The only effective way to reduce this problem is to
>> reduce writes to the boot-sector.
> exFAT's specification defect... Well..
> Even though the boot sector is corrupted, It can be recovered using the backup boot sector
> through fsck.

Exactly.
However, in order to execute fsck, it is necessary to recognize the partition/volume with broken boot-sector as exfat.
Can linux(or fsck) correctly recognize the FileSystem even if the boot-sector cannot be read?
(I don't yet know how linux recognizes FileSystem)
In fact, a certain system recognize it as 'Unknown format'.
Nowadays, exfat is often used for removable storage.
This problem is not only for linux.

BR
---
etsuhiro Kohada <kohada.t2@...il.com>