Cluster outage with old-gen size increasing

Hi,
We are seeing logs like this in our ElasticSearch 6.7:
[gc][young][678215][290288] duration [718ms], collections [1]/[1s], total [718ms]/[4.5h], memory [9.6gb]->[10.3gb]/[30.1gb], all_pools {[young] [108.4mb]->[332.8mb]/[998.5mb]}{[survivor] [124.7mb]->[124.7mb]/[124.7mb]}{[old] [9.3gb]->[9.8gb]/[29gb]}
The timing coincides with a cluster outage. What does the above log mean/indicate? Why the garbage collection is taking long?
-Musfiqur