Why is there storage limit in Cloud Bigtable?

Google's Cloud Bigtable uses their Colossus filesystem to store data. I thought this would mean an instance with even a single Node could handle any size of data (assuming of course there is little enough reads and writes that Nodes CPU can handle it).

Even in documentation it states:
> Importantly, data is never stored in Bigtable nodes themselves; each node has pointers to a set of tablets that are stored on Colossus.

I could imagine an application where Node(s) handle data stream from many IoT devices and store them in Bigtable. Most of that data would be rarely accessed, thus most of Node's CPU work would be in storing new data in Bigtable, and only some in accessing very old entries.

Yet, in quotas documentation one could learn that there is a per Node storage limit:
> If a cluster does not have enough nodes, based on its current workload and the amount of data it stores, Bigtable will not have enough CPU resources to manage all of the tablets that are associated with the cluster. Bigtable will also not be able to perform essential maintenance tasks in the background. As a result, the cluster may not be able to handle incoming requests, and latency will go up. See Trade-offs between storage usage and performance for more details.

> To prevent these issues, monitor storage utilization for your clusters to make sure they have enough nodes to support the amount of data in the cluster, based on the following limits:
>- SSD clusters: 5 TB per node
>- HDD clusters: 16 TB per node

> ...

> Important: If any cluster in an instance exceeds the hard limit on the amount of storage per node, writes to all clusters in that instance will fail until you add nodes to each cluster that is over the limit.

I don't understand where this limitation is coming from. Just because some data is laying still in Colossus should add CPU usage to a Node, nor occasional background moving around few terabytes of data shouldn't be a problem for today's hardware? I thought the idea of using Colossus was to allow for limitless storage, yet the limitation suggest nodes could just use own storage without any smart networking filesystem.

Or am I missing something and one can configure a single Cluster and single Node instance that will be able to access e.g. 100TB of data? And the limits are for some internal Node cache?

Cloud Bigtable has storage limits due to performance and operational reasons, not hardware constraints. These limits ensure high performance, balanced data distribution, and efficient maintenance. Exceeding them may affect reliability and scalability.

Reasons for storage limits in Cloud Bigtable:

1. Storage limits ensure that each node can efficiently manage and handle the data it contains, maintaining good performance.

2. Setting storage limits prevents a single node from becoming overloaded with excessive data, which could negatively impact system performance and scalability.

3. Too much data per node could make these tasks resource-intensive and affect overall performance.

4. Limiting data per node enables easy replication for data redundancy and fault tolerance.

Always refer to the latest documentation to stay updated on Cloud Bigtable's storage limits.