[Gluster-devel] Question on choosing source of replica to heal with AFR

[Ravishankar N]

On 02/28/2014 07:28 AM, Zhang Huan wrote:
> Hello Ravi,
>
> Thanks for your reply.
>
> Sorry that I have a typo in my mail. It should by "underlying 
> corruption" instead of "underlying correction".
>
> I guess the logic of eliminating zero byte files from all innocent 
> nodes is working for preventing underlying corruption to propagate to 
> other brick. Asked in another way, if the underlying brick finds some 
> file is corrupted, anything it could do to tell glusterfs to fix it?
>
Hi Zhang,
If all nodes are innocent (from AFR's point of view) ,then AFR cannot  
use the changelog attributes to determine which is source. In this case, 
the safest bet is to mark all zero byte files as sink, so that we don't 
end up healing in the wrong direction.  Like I said earlier, AFR can 
only use the changelog attributes (xattrs) to determine the 
source/sinks. It cannot detect underlying on disk file system 
corruptions outside the scope of the xattrs.

If you are sure that a particular brick is the right source despite the 
xattrs saying otherwise, you can manually change the attributes of the 
file on all bricks so that AFR now sees that brick as the source and 
heals in the expected direction.

-Ravi

> Zhang Huan
>
>
> On Thu, Feb 27, 2014 at 7:50 PM, Ravishankar N <ravishankar at redhat.com 
> <mailto:ravishankar at redhat.com>> wrote:
>
>     On 02/26/2014 07:42 PM, Zhang Huan wrote:
>>     Hello guys,
>>
>>     Anyone know about my question?
>>
>>     Zhang Huan
>>
>>
>>     On Sun, Feb 23, 2014 at 11:28 AM, Zhang Huan <zhhuan at gmail.com
>>     <mailto:zhhuan at gmail.com>> wrote:
>>
>>         Hello all,
>>
>>         While reading codes about how to choose healing source, there
>>         is one thing that confuse me. Say we have 3 replica, and 2 of
>>         them are OK and the left one is outdated due to temporary IO
>>         failure. For some reason, one of the 2 correct replica is
>>         truncated to 0 due to some underlying correction. Will
>>         glusterfs kick the 0 size file out? or still consider it a
>>         correct one and may corrupt the left correct replica by healing?
>>
>     Out of the two correct replicas, gluster will pick the first
>     healthy replica brick as source [see afr_sh_select_source()]. If
>     that brick is truncated at the back-end due to 'underlying
>     correction' (not sure what that means), then yes I'm afraid it
>     will still be considered as correct source and you would get zero
>     byte file in other 2 bricks because of the healing.
>
>>         In function afr_mark_sources(), it kicks 0 size file out when
>>         all nodes are innocent. Even when all nodes are fools, the
>>         file with largest size will be chosen as source. When it
>>         comes to the case that there is wise nodes, it won't further
>>         check file size. Considering different file size of replicate
>>         will trigger healing to work, I am wondering if there is any
>>         reason behind the code?
>>
>     The changelog extended attributes are marked  by AFR based on the
>     result of whether the file operation succeeded or not on each of
>     the replica. It uses those attributes to determine the
>     source/sink. Direct modification of the file at the brick will
>     invalidate any meaning that the changelog holds.
>     Thanks,
>     Ravi
>
>>
>>         Thanks.
>>
>>         Zhang Huan
>>
>>
>>
>>
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>
>

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