[linux-lvm] corruption on reattaching cache

[Zdenek Kabelac]

Dne 10.6.2016 v 12:51 Markus Mikkolainen napsal(a):
>
> could there atleast be some kind of a warning if the reattaching will result
> in you destroying your filesystem? or possibly make the "--splitcache" warn
> that this is a debug feature and the result might be bad?
>

I'll try to provide some fixing commit to let reattach without
zeroing require  -Zn with lvconvert.


> basically NOTHING suggested to me that i might be doing something that could
> destroy my filesystem.

Yep

>
>> Dne 9.6.2016 v 22:24 Markus Mikkolainen napsal(a):
>>>  I seem to have hit the same snag as Mark describes in his post.
>>>
>>>  https://www.redhat.com/archives/linux-lvm/2015-April/msg00025.html
>>>
>>>  with kernel 4.4.6 I detached (--splitcache) a writeback cache from a
>>>  mounted
>>>  lv which was then synchronized and detached. Then I reattached it and
>>>  shortly
>>>  detached it again. What was interesting is that after the second detach it
>>>  synchronized AGAIN starting from 100% , and then I started getting
>>>  filesystem
>>>  errors. I immediately shutdown, and forced an fsck , and didnt lose that
>>>  much
>>>  data, but still had some stuff to correct.
>>>
>>>  It looked to me like a detached cache, being reattached will retain all
>>>  cached
>>>  data on it, even though it was supposed to be written to the backing disk,
>>>  and
>>>  then instead of marking it clean on attaching, it will continue serving
>>>  old
>>>  data from the cache.
>>>
>>
>>
>> Yes - known issue,  --splitcache is rather for 'debugging' purposes.
>> Use --uncache  and create new cache when needed.
>>
>> Splitted cache needs to be cleared on reattachment - but that needs further
>> code rework.
>>
>> The idea behind is - we want to support 'offline' writeback of data as ATM
>> cache target doesn't work well if there is any disk error - i.e. cache is in
>> writeback mode and has 'error' sector - you can't clean such cache...
>>
>> Regards
>>
>> Zdenek
>>
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>>
>
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