Another single malt served up..

 I had this blog written up and saved a while back...in fact 5 months ago.  I thought I would wrap it up and put out.  It''s better late than never.  So here it goes.

The team working on this new functionality that improves disaster recovery chose the name Ledaig since it was the easiest to pronounce.  After a project name like Bunnahabhain its hard to come up with something bigger.  Ledaig picks up where Ardbeg left :-)  A 10,000 ft view of  Ledaig is that it allows a user to import a diskset even when some of the disks have failed.  However this description barely conveys the complexity and the effort required to do it.

The Ardbeg project introduced metaimport.  Given limited resources, it was decided that it would deal with only fully functioning disksets.  That is   disksets could not have any drive in an errored state.  This of course is really limiting and this led to the project internally called Ledaig.  The complexity of this project arises from the combinatorial set of events that need to be resolved in order to import a diskset.  For example, if a metadb on one of the drive has failed,  then during the scan that needs to be eliminated.  But extend that to 4 drives where half of the metadbs are dead.  Now things become interesting.  Another problem occurs if some drives have moved from one diskset to another or if one has simply thrown together disks with stale metadbs. 

The code considers all these options and does its best to come up with a sane combination.  However, when it cannot resolve it, it provides the user with enough data to help make the correct choice.  Also as part of this project a number of bugs in metaimport were fixed.   So far this functionality is in Nevada.


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