[Buildbot] #3129: Document migration from one database engine to another

[Buildbot trac]

#3129: Document migration from one database engine to another
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Reporter:  sa2ajj  |      Owner:
    Type:  task    |     Status:  new
Priority:  major   |  Milestone:  0.9.+
 Version:  master  |   Keywords:  docs
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 Buildbot currently supports a number of database engines through
 SQLAlchemy.

 It is quite usual for new users to use SQLite since it requires no
 complicated setup, however with current change to Data API the amount of
 information stored in the database is much bigger than it used to be,
 which might trigger a desire to move to a different SQL database.

 We do not seem to have instructions on how to perform this migration
 though.  This ticket is about writing such documentation.

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Ticket URL: <http://trac.buildbot.net/ticket/3129>
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