Re: ubifs: why ubifs doesn't support 2 copied of super blocks for better fs reliability

From: Richard Weinberger <richard@nod.at>
To: chengzhihao1 <chengzhihao1@huawei.com>
Cc: Ryder Wang <rydercoding@hotmail.com>,
	 linux-mtd <linux-mtd@lists.infradead.org>
Subject: Re: ubifs: why ubifs doesn't support 2 copied of super blocks for better fs reliability
Date: Sat, 29 Jun 2024 15:56:29 +0200 (CEST)	[thread overview]
Message-ID: <84530892.9887.1719669389649.JavaMail.zimbra@nod.at> (raw)
In-Reply-To: <f9c0f1f3-5fbf-c86d-4056-b75ee368df76@huawei.com>

----- Ursprüngliche Mail -----
> Von: "chengzhihao1" <chengzhihao1@huawei.com>
> An: "Ryder Wang" <rydercoding@hotmail.com>, "linux-mtd" <linux-mtd@lists.infradead.org>, "richard" <richard@nod.at>
> Gesendet: Samstag, 29. Juni 2024 13:22:34
> Betreff: Re: ubifs: why ubifs doesn't support 2 copied of super blocks for better fs reliability

> 在 2024/6/28 10:28, Ryder Wang 写道:
>> Hello,
>> 
>> It looks ubifs has only 1 super block. Super block may be corrupted by power-cut

No. The super block must not get corrupted by a sudden loss of power.
If you're facing this, something else in your storage stack is broken.

>> while NAND erasing or writing, ECC error and etc. When super block is
>> corrupted, ubifs mount will always fail.
>> 
> 
> Have you met the problem caused by corrupted superblock? I only saw once
> in our product, in which there ware many problem eraseblocks in the flash.
> 
>> I am asking why ubifs doesn't support 2 copied of super blocks for better fs
>> reliability? As we know, both uibfs master block and ubi volume table have 2
>> copies, so they are very stronger. Is there any dev plan to support it on super
>> block?
> 
> I have no ideas about this question either after looking through the
> history and whitepaper[1]. The superblock is updated in a very low
> frequency(eg. auto-resize), and it is updated by atomic changing. So I
> guess that corrupted superblock is hardly generated from the view of
> UBIFS layer. However, the UBI will do wear-leveing, which could
> atomically exchange two LEBs(may contain superblock), then the updating
> frequency of superblock could be increased. The final probability of
> corrupted superblock is hard to say. So, I'm not sure it's a very
> valuable thing to maintain two superblocks in UBIFS, afterall, it will
> change disk structure.

UBIFS relies on working flash memory, so having multiple copies makes no sense
since one of UBIFS' goals is also being space efficient.
There is one exception: it has two master LEBs. For historic reasons,
it needs to exchange theirs contents automatically, at the time when UBIFS
was young, UBI had no atomic-leb-change feature.

Thanks,
//richard

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