[HPDD-discuss] Removal of SOM code

The Size on MDT (SOM) feature has been in a prototype state for several years, with no signs of moving beyond this prototype stage. Several problems exist in the code today, primarily that recovery is not really implemented, yet the existing code adds complexity on the clients and servers. Without proper recovery, the current code risks file data loss if the SOM data isn't updated on the MDS consistently with data writes to the OST. We're planning to remove the SOM code from the master branch as a result, tracked under https://jira.hpdd.intel.com/browse/LU-6047: - http://review.whamcloud.com/13126 - http://review.whamcloud.com/13169 - http://review.whamcloud.com/13442 - http://review.whamcloud.com/13443 Some of the performance improvements of SOM have been implemented by statahead. I think a case could be made for a very stripped down SOM to be implemented in the future, that only deals with single-client writers and synchronously invalidates the file size on open-for-write, which isn't so bad with flash storage for the MDT as is typical today. The size of files that do not get set at initial write or are invalidated by an open can be updated asynchronously by LFSCK doing a periodic scan in the background. Since this stripped-down implementation would have very little to do with the current implementation, there isn't much benefit to even trying to fix the current code in place. I definitely prefer presenting about new features going into Lustre, but I also think it is important that people are aware when a semi-feature like this is being removed. I don't believe that anyone is actually using this feature today, and the reduction in code maintenance and complexity will help both ongoing maintenance and bug fixing, as well as make it a that much easier for new developers to understand the code. Cheers, Andreas -- Andreas Dilger Lustre Software Architect Intel High Performance Data Division