nicoloboschi commented on issue #22051:
URL: https://github.com/apache/pulsar/issues/22051#issuecomment-1964171900

   Hi @MonicaMagoniCom @lhotari
   Even if we ensure that the bookies are still on BK 4.14, at some points user 
will have to perform the upgrade to newer Pulsar/BK versions. 
   
   IIUC the write format used by BK 4.15, is not compatible with BK 4.14 (I'm 
referring to server versions). I don't think this is acceptable neither for 
Pulsar and BookKeeper.
   
   I believe @hangc0276 found the proper way to upgrade/downgrade without 
issues in https://github.com/apache/bookkeeper/pull/3793
   
   From my understanding it should be done like this:
   - before upgrading to 3.0, set ´dbStorage_rocksDB_checksum_type=kCRC32c´ in 
the bookies
   - upgrade bookies -> now the bookies should continue writing in format 
`kCRC32c`, avoiding the new default `kXXH3`
   - downgrade bookies should be fine since the format didn't change
   
   
   But at the same time, BK uses two different formats for different purposes 
   - ledger index -> kCRC32c
   - entry log location-> kxxHash
   
   but the new setting would set both. Since they're different that would end 
up in a breaking change in one of the two.
   So there's no really way at the moment to keep full compatibility
   
   @hangc0276 Am I correct? 


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