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From Anuj Wadehra <anujw_2...@yahoo.co.in>
Subject Incremental Repair
Date Sun, 12 Mar 2017 17:44:41 GMT
Hi,

Our setup is as follows:
2 DCS with N nodes, RF=DC1:3,DC2:3, Hinted Handoff=3 hours, Incremental Repair scheduled once
on every node (ALL DCs) within the gc grace period.

I have following queries regarding incremental repairs:

1. When a node is down for X hours (where x > hinted handoff hours and less than gc grace
time), I think incremental repair is sufficient rather than doing the full repair. Is the
understanding correct ? 

2. DataStax recommends "Run incremental repair daily, run full repairs weekly to monthly".
Does that mean that I have to run full repairs every week to month EVEN IF I do daily incremental
repairs? If yes, whats the reasoning of running full repair when inc repair is already run?

Reference: https://docs.datastax.com/en/cassandra/3.0/cassandra/operations/opsRepairNodesWhen.html

3. We run inc repair at least once in gc grace instead of the general recommendation that
inc repair should be run daily. Do you see any problem with the approach? 

As per my understanding, if we run inc repair less frequently, compaction between unrepaired
and repaired data wont happen on a node until some node runs inc repair on the unrepaired
data range. Thus, there can be some impact on disk space and read performance but immediate
compaction is anyways never guaranteed by Cassandra. So, I see minimal impact on performance
and that too just on the reads of delta data generated between repairs. 

Thanks
Anuj

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