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Subject: Re: Small fs
To: Henk Slager <eye1tm@gmail.com>
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Cc: Martin Steigerwald <martin@lichtvoll.de>,
        Imran Geriskovan <imran.geriskovan@gmail.com>,
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From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
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Date: Mon, 12 Sep 2016 10:12:57 -0400
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On 2016-09-12 10:09, Henk Slager wrote:
>> FWIW, I use BTRFS for /boot, but it's not for snapshotting or even the COW,
>> it's for DUP mode and the error recovery it provides.  Most people don't
>> think about this if it hasn't happened to them, but if you get a bad read
>> from /boot when loading the kernel or initrd, it can essentially nuke your
>> whole system.  I run BTRFS for /boot in DUP mode with mixed-bg (because I
>> only use 512MB for boot) to mitigate the chance that a failed read has any
>> impact, and ensure that if it does, it will refuse to boot instead of
>> booting with a corrupted kernel or initrd.
>
> Suppose kernel and initrd are on a BTRFS fs with data, metadata and
> system all single profile. Will a bootloader then just continue
> booting up a system even when there are csum errors in kernel and/or
> initrd files?  Suppose the bootloader is grub2.
>
Assuming I understand how they implemented their BTRFS support in GRUB, 
it should still be checking csums just like the kernel implementation, 
and thus should throw a read error when a block is corrupted.  For a 
stock upstream version of GRUB2, this would mean that it will refuse to 
boot that entry, and may try to auto-boot a fallback entry (which will 
probably fail given how most distros set up their fallback entries).