linux-kernel - Re: limits on raid

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Message-ID: <467AAEB3.9070804@rtr.ca>
Date:	Thu, 21 Jun 2007 13:00:35 -0400
From:	Mark Lord <lkml@....ca>
To:	david@...g.hm
Cc:	David Chinner <dgc@....com>, Neil Brown <neilb@...e.de>,
	Avi Kivity <avi@...o.co.il>, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: limits on raid

david@...g.hm wrote:
> On Thu, 21 Jun 2007, David Chinner wrote:
> 
>> On Thu, Jun 21, 2007 at 12:56:44PM +1000, Neil Brown wrote:
>>
>>> I have that - apparently naive - idea that drives use strong checksum,
>>> and will never return bad data, only good data or an error.  If this
>>> isn't right, then it would really help to understand what the cause of
>>> other failures are before working out how to handle them....
>>
>> The drive is not the only source of errors, though.  You could
>> have a path problem that is corrupting random bits between the drive
>> and the filesystem. So the data on the disk might be fine, and
>> reading it via a redundant path might be all that is needed.
> 
> one of the 'killer features' of zfs is that it does checksums of every 
> file on disk. so many people don't consider the disk infallable.
> 
> several other filesystems also do checksums
> 
> both bitkeeper and git do checksums of files to detect disk corruption

No, all of those checksums are to detect *filesystem* corruption,
not device corruption (a mere side-effect).

> as david C points out there are many points in the path where the data 
> could get corrupted besides on the platter.

Yup, that too.

But drives either return good data, or an error.

Cheers
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