What are the implications of using a low GCGraceSeconds value in Cassandra?

Question

The default setting of Cassandra for GCGraceSeconds is 10 days.

I wonder why this is defaulted that big and what the implications will be if this will be a lower number like 60 seconds.

The reason for asking is that I want to achieve a faster "cleaning" of ghost rows that pop up after their removal. I am often reading these rows with some scans and need to put some logic into the client to validate that the content is not deleted yet.

How can a lower GCGraceSeconds value effect the system and how does it compare to having this kind of ghost rows for some days?

Answer 1

GCGraceSeconds puts an upper bound on the amount of time the cluster had to propagate tombstones. A low GC grace seconds means than if a host is not contactable for that period, and doesn't receive the tombstone, then a deleted value could "come back from the dead".

It is advised that you run repair at least every GCGraceSeconds, so you can guarantee that the tombstone have been replicated. With a very small value for this, you will not be able to run repair often enough.