Hi @Theseeker
Media agents are indexing version agnostic - they can all handle both V1 or V2 indexes at the same time, so no issues there. Typically new clients will default to using V2, whereas old clients may need to go through an upgrade procedure to get them to V2, or a new pseudoclient has to be created in some cases (i.e for VMware/virtualization clients). Also, check out the available reports as they can be really helpful.
With V2, each backup job (full, diff or incremental) will generate ‘action logs’ - this is the index for the current job, and the current job only. These action logs are protected in each job, similar to V1. If you lost your index, and did not have access to an index backup, Commvault can use all the action logs to reconstruct the index database from all the jobs - however, this is time-consuming and for large backupsets it can take a while before you can begin your restore. In contrast, V1 indexing was a FULL index database backup included with each and every job. That used considerably more space and slowed down operations because we’d have to maintain a larger database synchronously, but the benefit was quick recovery should you lose your index.
Fortunately, backing up action logs with V2 is only a failsafe - the index backup happens at the “big apps” level as you noted. This is an independent backup and will additionally backup the full index database for each subclient/backupset (depending on the agent). If the index is lost, this backup allows for quick and seamless recovery without having to reconstruct much at all (perhaps only the last few jobs after the backup was done).
So if you need to restore 6 months ago, that should be in the active V2 database on the media agent without issue. If you lost your index cache and wanted to restore from 6 months ago, then the index backup is automatically restored, and any outstanding transaction logs are applied to make it operational for browse & restore - but this is a very unlikely scenario unless you have a DR situation.
If you are using tape then you certainly want to ensure your indexing backups are successful. If the index backups are not available then you can still restore, but it's going to need to pull at least a full cycle worth of tapes to reconstruct the index, which is not ideal. There is logic to ensure a copy of the indexing backups are available everywhere your backups are for this very scenario.
I am not sure if we will write V2 index backups to selective copies - I think that was planned but not sure about the current status. Will need to check internally ...