Re: Why not just return an error?

[keld@keldix.com]

On Fri, Oct 07, 2016 at 02:32:40AM +0300, Dark Penguin wrote:
> Greetings!
> 
> The more I read about md-raid, the more I notice that the biggest 
> problem of it: if you hit an error on a degraded RAID, it falls apart. 
> Because of this, it is possible to lose a huge amount of data due to one 
> tiny read error, which particularly makes raid5 the sword of Damocles.
> 
> But one question keeps me increasingly frustrated. Yes, during its 
> normal functioning, it totally makes sense to kick a faulty device out 
> of an array. But if we're running a degraded array, and doing so will 
> definitely result is massive data loss, why not just return a read error 
> instead? Just add a little check: on error, if degraded -> then just 
> return an error. I believe this is the dream of everyone who had ever 
> dealt with RAIDs.
> 
> With RAID, the first proprity is keeping data safe. Yes, it's not an 
> alternative to backups and all that, but still - if we hit an error on a 
> degraded array, the array should scream and panic and send all kinds of 
> warnings, but definitely NOT collapse and warrant a visit to the RAID 
> recovery laboratory (or this mailing list). Imagine how much headache 
> and lost hair would that relieve!..
> 
> Now, I'm probably not the first one to think of such a bright idea. So 
> there must be a very good reason why this is not possible; I don't think 
> the problem is just that "the existing behaviour is preferred, and 
> anyone who does not agree is an idiot". If not for enterprise use, then 
> at least it would be very useful for the "home archive" scenario when 
> "uptime" and "absense of errors" hold much less meaning than "losing one 
> file and not all the data". So, why is this not possible?..

Likewise, when the first disk fails, one could mark it as kind of in an error state,
and keep it running, and if one gets a read error, then you could get
the data from the good disks.

Often read errors can be remedied by writing data to the failing disk.
The good data could then be obtained from the good parts of the array.

This behaviour could be optional and could even be set during operation.

Best regards
keld