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

[Zhang Huan]

On Fri, Feb 28, 2014 at 12:07 PM, Ravishankar N <ravishankar at redhat.com>wrote:

>  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
>

Hello Ravi,

IMO, changing the attributes might be dangerous, since concurrent access
with glusterfs is introduced. Not sure if glusterfs has already provided
some mechanism for this.

My suggestion is to eliminate the zero-byte file from heal source even if
is marked as a source. If the underlying filesystem finds some corruption
(by scrubbing daemon after checking data checksum), it could truncate it to
0 and let glusterfs to do the healing job. Here is several cases of
analysis in my mind.
1. If this corrupted file is marked as the only source, then there is no
correct replica in the filesystem (actually all are fools), just pick any
one as the source to heal is OK;
2. If the corrupted file is one of the potential sources, eliminate this
one should keep healing in the right direction without further corrupting
other correct replicas.
3. If the corrupted file is not marked as a source, some other replica will
be chosen as a source and this file will be overwritten with correct data.
4. If there is no one is marked as clean by attribute, it is quite unlikely
this file is chosen as a source as its size is 0. Even it is chosen as a
source, there is no further corruption of file content after heal.

Zhang Huan
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