docs: add a reference to https://github.com/shazow/sqlalchemygrate for migration from sqlite to other dbs

docs/usage/performance.rst

@@ -1,63 +1,66 b''
 .. _performance:
 
 ================================
 Optimizing Kallithea Performance
 ================================
 
 When serving a large amount of big repositories, Kallithea can start
 performing slower than expected. Because of the demanding nature of handling large
 amounts of data from version control systems, here are some tips on how to get
 the best performance.
 
 * Kallithea will perform better on machines with faster disks (SSD/SAN). It's
   more important to have a faster disk than a faster CPU.
 
 * Slowness on initial page can be easily fixed by grouping repositories, and/or
   increasing cache size (see below). This includes using the lightweight dashboard
   option and ``vcs_full_cache`` setting in .ini file
 
 
 Follow these few steps to improve performance of Kallithea system.
 
 
 1. Increase cache
 
     In the .ini file::
 
      beaker.cache.sql_cache_long.expire=3600 <-- set this to higher number
 
     This option affects the cache expiration time for the main
     page. Having several hundreds of repositories on main page can
     sometimes make the system behave slowly when the cache expires for
     all of them. Increasing the ``expire`` option to a day (86400) or a
     week (604800) will improve general response times for the main
     page. Kallithea has an intelligent cache expiration system and it
     will expire the cache for repositories that have been changed.
 
 2. Switch from sqlite to postgres or mysql
 
     sqlite is a good option when having a small load on the system. But due to
     locking issues with sqlite, it is not recommended to use it for larger
     deployments. Switching to mysql or postgres will result in an immediate
-    performance increase.
+    performance increase. A tool like SQLAlchemyGrate_ can be used for
+    migrating to another database platform.
 
 3. Scale Kallithea horizontally
 
     Scaling horizontally can give huge performance increases when dealing with
     large traffic (large amount of users, CI servers etc). Kallithea can be
     scaled horizontally on one (recommended) or multiple machines. In order
     to scale horizontally you need to do the following:
 
     - Each instance needs its own .ini file and unique ``instance_id`` set.
     - Each instance's ``data`` storage needs to be configured to be stored on a
       shared disk storage, preferably together with repositories. This ``data``
       dir contains template caches, sessions, whoosh index and is used for
       task locking (so it is safe across multiple instances). Set the
       ``cache_dir``, ``index_dir``, ``beaker.cache.data_dir``, ``beaker.cache.lock_dir``
       variables in each .ini file to a shared location across Kallithea instances
     - If celery is used each instance should run a separate Celery instance, but
       the message broker should be common to all of them (e.g.,  one
       shared RabbitMQ server)
     - Load balance using round robin or IP hash, recommended is writing LB rules
       that will separate regular user traffic from automated processes like CI
       servers or build bots.
+
+.. _SQLAlchemyGrate: https://github.com/shazow/sqlalchemygrate