Cassandra Migration questions

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I have the following Cassandra migration related questions so I have a better understanding of exactly what is going on in the background while a long migration is in progress.

  1. Once a migration has started does DM start using Cassandra immediately for all new trending and alarms or does it continue to use MySQL until the Migration has completed, which could be many days?
  2. In relation to #1 above, during a long migration, where does DM pull data from for trend graphs and alarm reports?
  3. Once the system is using Cassandra are the system cache Failover and Failover sync folder still used at all, and if yes what for?
  4. A while after my migration was completed I noticed in C:\Skyline DataMiner\System Cache\Failover” a number of sql query files with current date/timestamp. I looked at one and it contained a few delete of rp_pd_*** table commands. The files then disappeared a few minutes later, presumably after they got processed. If the system was running cassandra why was it generating SQL query commands to be processed

Thanks in advance for any details provided.

Marieke Goethals [SLC] [DevOps Catalyst] Selected answer as best 14th July 2023

1 Answer

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Hi Jeff,
assuming this is about MySQL to Cassandra single migration, which you can do trough cube:

  1. Writing data will only happen towards Cassandra, with the exception of removing and truncating data.
  2. The data will be read from both MySQL & Cassandra and be squashed into one response.
  3. Since no data is written to MySQL anymore, nothing will be synced, except for deletes & truncates.
  4. This will happen untill a restart is done. Once the migration is marked as completed, reads will only happen from cassandra, but deletes and truncates will also happen on mysql.

Hope this clarifies somethings.

Marieke Goethals [SLC] [DevOps Catalyst] Selected answer as best 14th July 2023
Jeff Douglass commented

Bert, yes this is for migrating a single or Failover DMA pair from MySQL to Cassandra. Thanks for response, this is exactly the info I was looking for. Just for clarification of response #4, so the DMA will continue to attempt to perform the MySQL cleanup tasks until the DMA(s) are restarted and the DMAs are not restarted automatically after the migration is complete?
1) What happens if the SLDMADB database is removed and/or MySQL is shut down are removed?
2) Are all the usual dbase cleanup tasks still executed until a restart? Queries like attempting to delete all 5 min trending data older than say 30 days?

Thanks again.
Bert Vervaele [SLC] commented

1) First of all I would definitly not advise deleting/stopping it before a restart has happend. But I believe it would show an alarm about you DB being down and trying to revive the service.
2) And yes TTL cleaning will happen until a restart.