[linux-lvm] corruption on reattaching cache

[Markus Mikkolainen]

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?

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

> 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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