We're in a process of migration from older hardware under ClickHouse to the newer generation.
Older machines have 12x6T disks, 128GB RAM and 2x10G NICs, newer machines have 12x10T disks, 256GB RAM and 2x25G NICs. Dataset per replica is around 35TiB. Each shard is 3 replicas.
Our process is:
The issue we're seeing is that source replicas saturate disks, starving user queries and merges.
It takes ~7h to replicate full dataset, below are the graphs for 12h around that time:
Naturally, source peers are not great at IO in the first place (that's why we're upgrading), but having 7h of degraded service is not great. It'd be nice to be able to set recovery speed, so it doesn't starve other activities like user queries and merges.
Moreover, max number of parts in partition quickly reaches the max (we set to 1000) on the target replica, where inserts are throttled, which doesn't make things any better. With throttled recovery at lower speed but with higher duration, this will probably be even longer period. Maybe we should split threads that do merges and threads that do replication, it seems like whole pool is busy just replicating.
It is also possible that we're just doing it wrong, then it'd be great to have a guide describing the process.
The text was updated successfully, but these errors were encountered:
If anyone stumbles on this issue, we worked around it by throttling 50G interface to 4G-8G with
Linux is still not great at throttling ingress, so this has to be run on source replicas.