Hi Hakan, > I was also wondering if the max thread size, which is currently 5 in 9.4+, could be configurable
Yes, this makes sense to have this configuration. See EnvUtils class, which is now the recommended way to get system properties or environment variables in Solr code. The behavior before 9.4 was to have up to 50 threads per nodes doing backup/restore. We figured out this was saturing disks, and the overall time taken to do a snapshot for a big collection was larger than with some sort of throttling. With 9.4+, it is supposed to have 5 threads per node doing backups/restores/splits (the core admin operations that are IO intensive). For some reason in your case, it seems the "cached" thread pool does not grow as expected. If you have 5 replicas or more queued to be backed up on a node, this node *should* have 5 threads running on this. Just to be sure, you trigger an *async* backup, correct? Could you give some numbers on your cluster? When you submit the backup command, how many collections/shards/replicas do you have? And how many per node?
