Re: [PATCH] btrfs: raid56: Use correct stolen pages to calculate P/Q

From: Goffredo Baroncelli <kreijack@inwind.it>
To: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Cc: Qu Wenruo <quwenruo@cn.fujitsu.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs: raid56: Use correct stolen pages to calculate P/Q
Date: Sat, 26 Nov 2016 14:12:56 +0100	[thread overview]
Message-ID: <cd7ee5cc-d384-7c9c-d656-cef2569b749a@inwind.it> (raw)
In-Reply-To: <20161125043119.GG8685@hungrycats.org>

On 2016-11-25 05:31, Zygo Blaxell wrote:
>>> Do you mean, read the corrupted data won't repair it?
>>>
>>> IIRC that's the designed behavior.
>> :O
>>
>> You are right... I was unaware of that....
> This is correct.
> 
> Ordinary reads shouldn't touch corrupt data, they should only read
> around it.  Scrubs in read-write mode should write corrected data over
> the corrupt data.  Read-only scrubs can only report errors without
> correcting them.
> 
> Rewriting corrupt data outside of scrub (i.e. on every read) is a
> bad idea.  Consider what happens if a RAM controller gets too hot:
> checksums start failing randomly, but the data on disk is still OK.
> If we tried to fix the bad data on every read, we'd probably just trash
> the filesystem in some cases.

I cant agree. If the filesystem is mounted read-only this behavior may be correct; bur in others cases I don't see any reason to not correct wrong data even in the read case. If your ram is unreliable you have big problem anyway.

The likelihood that the data contained in a disk is "corrupted" is higher than the likelihood that the RAM is bad.

BTW Btrfs in RAID1 mode corrects the data even in the read case. So I am still convinced that is the RAID5/6 behavior "strange".

BR
G.Baroncelli
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