Re: limits on raid

[Justin Piszcz <jpiszcz@lucidpixels.com>]

On Thu, 21 Jun 2007, Mattias Wadenstein wrote:

> On Thu, 21 Jun 2007, 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....
>
> In theory, that's how storage should work. In practice, silent data 
> corruption does happen. If not from the disks themselves, somewhere along the 
> path of cables, controllers, drivers, buses, etc. If you add in fcal, you'll 
> get even more sources of failure, but usually you can avoid SANs (if you care 
> about your data).
>
> Well, here is a couple of the issues that I've seen myself:
>
> A hw-raid controller returning every 64th bit as 0, no matter what's on disk. 
> With no error condition at all. (I've also heard from a collegue about this 
> on every 64k, but not seen that myself.)
>
> An fcal switch occasionally resetting, garbling the blocks in transit with 
> random data. Lost a few TB of user data that way.
>
> Add to this the random driver breakage that happens now and then. I've also 
> had a few broken filesystems due to in-memory corruption due to bad ram, not 
> sure there is much hope of fixing that though.
>
> Also, this presentation is pretty worrying on the frequency of silent data 
> corruption:
>
> https://indico.desy.de/contributionDisplay.py?contribId=65&sessionId=42&confId=257
>
> /Mattias Wadenstein
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Very interesting slides/presentation, going to watch it shortly.