Re: Why not just return an error?

[Dark Penguin <darkpenguin@yandex.ru>]

On 07/10/16 19:52, Phil Turmel wrote:
> Hi DP,
>
> {It's good that you are trimming replies, but don't cut the ID of who
> wrote what. }

Oh, yeah, sorry.


> You want to push the failure condition from being "broken raid with
> likely salvageable data, except for one sector" to "repeated errors to
> the upper layers with unknowable corruption as side effects".

That actually describes it pretty well, yes. %) Being able to choose a 
failure condition most suitable for your specific situation, and being 
able to push it that far and still have a working RAID if you want that.


> Then patch your kernel with your desired behavior.  "Free software"
> doesn't mean someone writes what you want for free.  And I disagree with
> you, so would object to it being put in the mainline kernel.

Yes, that's one of the things on my TODO list once I become a developer 
able to do that. :) I just thought I'm probably not the only one who 
wants that, and so I wanted to learn why is it not possible. And listen 
to what other people really think about it.


>> Anyway, if I had a collapsed RAID-5, I would want to at least have an
>> easy option to start it in a read-only mode in the last-known working
>> state, while the faulty drives are still not out of sync, and recover
>> data easily (to my single backup drive), or continue using the array for
>> a while, manually deleting one "bad" file if necessary; this is of
>> course not a "good thing" to do, but this way, RAID would be at least
>> not worse than single drives with faulty sectors, which are capable of
>> that, while RAIDs are not! I would be fine with that in my archive - as
>> I'm fine with some less importand parts of the archive being on faulty
>> single drives. It's just that I don't want to lose the whole drive due
>> to a hardware failure - and RAID adds more causes other than that,
>> instead of offering more protection against that.
>
> MD raid has no idea what is at any given sector.  And with a
> near-infinite variety of layering choices, there's no way it's going to.
>   That's why *you* have to do this.  You trimmed my description of the
> only "easy option" actually trustable.

I actually wanted to ask about that. Can you really ddrescue a drive 
with a "hole" in it, re-add it and expect it to work?.. What happens if 
you try to read from that "hole" again? And while I'm talking about 
re-adding, when does it become impossible to "re-add" a drive?..


-- 
darkpenguin