Re: unable to mount btrfs pool even with -oro,recovery,degraded, unable to do 'btrfs restore'

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-04-08 14:30, Chris Murphy wrote:
> On Fri, Apr 8, 2016 at 12:18 PM, Austin S. Hemmelgarn
> <ahferroin7@gmail.com> wrote:
>> On 2016-04-08 14:05, Chris Murphy wrote:
>>>
>>> On Fri, Apr 8, 2016 at 5:29 AM, Austin S. Hemmelgarn
>>> <ahferroin7@gmail.com> wrote:
>>>
>>>> I entirely agree.  If the fix doesn't require any kind of decision to be
>>>> made other than whether to fix it or not, it should be trivially fixable
>>>> with the tools.  TBH though, this particular issue with devices
>>>> disappearing
>>>> and reappearing could be fixed easier in the block layer (at least, there
>>>> are things that need to be fixed WRT it in the block layer).
>>>
>>>
>>> Another feature needed for transient failures with large storage, is
>>> some kind of partial scrub, along the lines of md partial resync when
>>> there's a bitmap write intent log.
>>>
>> In this case, I would think the simplest way to do this would be to have
>> scrub check if generation matches and not further verify anything that does
>> (I think we might be able to prune anything below objects whose generation
>> matches, but I'm not 100% certain about how writes cascade up the trees).  I
>> hadn't really thought about this before, but now that I do, it kind of
>> surprises me that we don't have something to do this.
>>
>
> And I need to better qualify this: this scrub (or balance) needs to be
> initiated automatically, perhaps have some reasonable delay after the
> block layer informs Btrfs the missing device as reappeared. Both the
> requirement of a full scrub as well as it being a manual scrub, are
> pretty big gotchas.
>
We would still ideally want some way to initiate it manually because:
1. It would make it easier to test.
2. We should have a way to do it on filesystems that have been 
reassembled after a reboot, not just ones that got the device back in 
the same boot (or it was missing on boot and then appeared).