Revision ef79eb82 doc/design-2.0-os-interface.rst

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- Input parameters: in 1.2 those were passed on the command line, in 2.0 we'll
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  use environment variables, as there will be a lot more information and not
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  all OSes may care about all of it.
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- Input/Output: in 1.2 import scripts accepted the instance contents from
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  standard input, and export scripts exported it to standard output. This can't
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  be done in 2.0 as we can have more than one disk and so we wouldn't have a
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  way to distinguish them. A target/source node and directory will be instead
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  passed, for the import script to do the correct job. All scripts will get
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  nothing in input and are supposed to output only user-relevant messages.
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- Number of calls: export scripts will be called once for each device the
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  instance has, and import scripts once for every exported disk. Imported
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  instances will be forced to have a number of disks greater or equal to the
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  one of the export.
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- Some scripts are not compulsory: if such a script is missing the relevant
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  operations will be forbidden for instances of that os. This makes it easier
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  to distinguish between unsupported operations and no-op ones (if any).
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OLD_INSTANCE_NAME


  		





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  rename: the name the instance should be renamed from.


  		





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EXPORT_NODE


  		





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  import/export: node where the export should be saved to or sourced from.


  		





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EXPORT_PATH


  		





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  import/export: the place where exports are/should be saved to.


  		





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EXPORT_COMPRESSION


  		





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  import/export: the type of compression for the exports.


  		





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(Rationale for the EXPORT_NODE, EXPORT_PATH decision: giving always a local


  		





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path would require either copying exports around, which takes a lot of


  		





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time/resources or depending on some type of remote filesystem to mount


  		





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resources on different nodes, which would add even more complexity to the


  		





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ganeti code, and can be added later, if really needed.)


  		





  
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EXPORT_DEVICE


  		





  
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  export: device to be exported, a snapshot of the actual device. The data must be exported to stdout.


  		





  
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EXPORT_INDEX


  		





  
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  export: sequential number of the instance device targeted.


  		





  
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IMPORT_DEVICE


  		





  
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  import: device to send the data to, part of the new instance. The data must be imported from stdin.


  		





  
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IMPORT_INDEX


  		





  
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  import: sequential number of the instance device targeted.


  		





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(Rationale for INSTANCE_NAME as an environment variable: the instance name is


  		





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always needed and we could pass it on the command line. On the other hand,


  		





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Output/Behaviour


  		





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~~~~~~~~~~~~~~~~


  		





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As discussed scripts should only send user-targeted information to stdout. The


  		





  
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As discussed scripts should only send user-targeted information to stderr. The


  		





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create and import scripts are supposed to format/initialise the given block


  		





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devices and install the correct instance data. The export script is supposed to


  		





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export instance data to a target node in a format understandable by the the


  		





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import script. The rename script should only modify the instance's knowledge of


  		





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what its name is.


  		





  
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export instance data to stdout in a format understandable by the the import


  		





  
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script. The data will be compressed by ganeti, so no compression should be


  		





  
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done. The rename script should only modify the instance's knowledge of what


  		





  
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its name is.


  		





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Other declarative style features


  		





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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


  		












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