Re: multi-device btrfs with single data mode and disk failure

[Alexandre Poux <pums974@gmail.com>]

Hi,

I finally did it : patched the kernel and removed the device.
As expected he did not scream since there was nothing at all on the device.
Now I'm checking that everything is fine:
scrub (in read only)
check (in read only)
but I think that everything will be OK
If not, I will rebuild the array from scratch (I did managed to save my
data)

Thank you both for your guidance.
I think that a warning should be put in the wiki in order for other user
to not do the same mistake I did :
never ever use the single mode

I will try to do it soon

Again thank you

Le 20/09/2016 à 23:15, Chris Murphy a écrit :
> On Tue, Sep 20, 2016 at 2:18 PM, Alexandre Poux <pums974@gmail.com> wrote:
>>
>> Le 20/09/2016 à 21:46, Chris Murphy a écrit :
>>> On Tue, Sep 20, 2016 at 1:31 PM, Alexandre Poux <pums974@gmail.com> wrote:
>>>> Le 20/09/2016 à 21:11, Chris Murphy a écrit :
>>>>> And no backup? Umm, I'd resolve that sooner than anything else.
>>>> Yeah you are absolutely right, this was a temporary solution which came
>>>> to be not that temporary.
>>>> And I regret it already...
>>> Well on the bright side, if this were LVM or mdadm linear/concat
>>> array, the whole thing would be toast because any other file system
>>> would have lost too much fs metadata on the missing device.
>>>
>>>>>  It
>>>>> should be true that it'll tolerate a read only mount indefinitely, but
>>>>> read write? Not sure. This sort of edge case isn't well tested at all
>>>>> seeing as it required changing the kernel to reduce safe guards. So
>>>>> all bets are off the whole thing could become unmountable, not even
>>>>> read only, and then it's a scraping job.
>>>> I'm not that crazy, I tried the patch inside a virtual machine on
>>>> virtual drives...
>>>> And since it's only virtual, it may not work on the real partition...
>>> Are you sure the virtual setup lacked a CHUNK_ITEM on the missing
>>> device? That might be what pinned it in that case.
>> In fact in my virtual setup there was more chunk missing (1 metadata 1
>> System and 1 Data).
>> I will try to do a setup closer to my real one.
> Probably the reason why that missing device has no used chunks is
> because it's so small. Btrfs allocates block groups to devices with
> the most unallocated space first. Only once the unallocated space is
> even (approximately) on all devices would it allocate a block group to
> the small device.
>
>