The Upgrading from an existing EBX® 6 environment section of the EBX 6.2.x release notes states that

"The upgrade process crawls all the records in the G_BLK database table to ensure their size is optimal by removing potential wasted space and splitting entries that became bigger than necessary. This one-shot process may significantly delay the first startup on large repositories."

Currently, we expect the migration to take time starting from 100 GB in the G_BLK table. 
Therefore, we recommend you to consider manual cleaning with the assistance of EBX support and engineering prior to upgrading to 6.2.x, particularly if its size exceeds 500 GB.
 
As a consequence, if your G_BLK meets the above conditions, prior to launching the 6.2.x upgrade, please create a support case providing the latest EBX kernel log file with the export of the  G_DTR, G_TRV, G_HR and G_TLR tables in order we can identify the table revisions responsible for retaining the blocs.

Upgrading to EBX 6.2.x can take a significant amount of time when the G_BLK table size is large.