Understanding Oak Index Handling by the Content Transfer Tool (CTT)

[Bhuwan_B]

We are preparing for a content migration from our current AEM instance to AEM as a Cloud Service using the Content Transfer Tool (CTT). While reviewing the documentation, we've noted the prerequisite that the total size of the Lucene Oak indexes on the source environment must not exceed 25 GB.

My query is about the technical reasoning for this specific limitation. We understand that the CTT is designed to migrate content (nodes and binaries), not the physical index data itself. On the target Cloud Service environment, content is re-indexed from scratch using the custom index definitions deployed via a Cloud Manager pipeline.

Given that the source indexes are not transferred, could someone clarify why this 25 GB size limit is imposed on the source system?

[SantoshSai · Accepted solution]

Hi @Bhuwan_B,

I think CTT still needs to traverse and serialize the full node structure under /oak:index during extraction. Even though the physical Lucene files themselves aren't transferred, the metadata and configurations in the index nodes can be quite large and deeply nested.

When this node structure exceeds 25 GB, it can cause:

• High heap memory usage on the source AEM instance,

• Performance bottlenecks or timeouts during extraction,

• And sometimes, the CTT job may fail altogether.

And I think, this limit is essentially a precautionary performance threshold to ensure the migration process runs smoothly without overwhelming the source environment.

Also, you can create a support ticket with Adobe Customer Care so you can discuss options for index size above this limit.

Reference: https://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/migration-journe...