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

[Zhang Huan]

On Fri, Feb 28, 2014 at 6:28 PM, Ravishankar N <address@hidden> wrote:

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.

If there is underlying FS corruption and we need to make gluster aware of it, then something like bit rot detection would be the way to go. You can find more information about some work in progress on the gluster website/ mailing list archives:
http://www.gluster.org/community/documentation/index.php/Arch/BitRot_Detection
http://lists.nongnu.org/archive/html/gluster-devel/2014-01/msg00209.html
https://lists.gnu.org/archive/html/gluster-devel/2014-01/msg00006.html

-Ravi

​Thanks for your material. I agree with the point from the discussion that glusterfs should make good use of checksum capability of the underlying filesystem (such as btrfs), for both simplicity and efficiency.

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.

​On the other hand, I think this might be a fix/improvment for the current status, for both adding further check to the underlying corruption and a simple way of underlying filesystem to tell glusterfs it has corruption in some file.

Zhang Huan