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Right now I have a 400GB database, with a 5 node Galera Cluster. They are all RAID 10 SSDs.

I've read the following:

If the node goes missing due to a network problem or otherwise leaves without telling the rest of the cluster, then problems can arise. For the cluster to function, it needs a quorum, a majority of nodes active in the cluster. The two other nodes will continue to function normally since their partition has more than half of the known nodes but the node that left will stop accepting queries when it realizes that it is no longer in contact with the active partition. In this case, assuming an application can access the two active nodes, the failure can go mostly unnoticed.

I am trying to reduce my cost and make things more optimal. My cluster is handling a few thousand queries per minute. Is it safe to have a 3 node cluster?

What happens if 1 or 2 of the nodes go down, would there be a total outage of the database?

Is it recommended to have a 5 node cluster over a 3 node cluster?

Should I put them on RAID 1, RAID 0, or RAID 10? What is suffice?

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If the criteria for "safe" is "no single point of failure", then 3 suffices.

If "safe" means surviving "any two points of failure", then there is no solution. 5 only handles the case where two servers go down, not arbitrary combinations of things.

RAID 10 (or 5, but not 1 or 0) provides recovery from a single disk failure on a single machine. Since a 3+ node cluster can survive the failure of one entire disk subsystem, RAID is not required; it just gives you an extra level of comfort.

I do like RAID 10 with Battery Backed Write Cache -- this has the bonus of making writes virtually instantaneous.

Here is a situation that can happen with a 3-node cluster (N1, N2, N3). Let's say N1 dies. After putting a new (or repaired) N1 into the cluster, it will rebuild the data. This uses N2 to as the 'donor' to rebuild N1. That leaves only N3 at full functionality. (N2 will be somewhat busy sending data to N1.) The Cluster is still alive, though "slowed".

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  • Thank you for this answer, it answers it perfectly - I was worried about the "quorum" being messed up in the case that 1 node goes offline. Should I be worried in the case that 2 nodes go down at the same time, a 5 node setup would save me? Or a 3 node setup should be solid too (I guess there would be 1 node left online still).
    – Kevin
    Jul 2, 2018 at 6:31
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    Are the 3 nodes in the same building? If so, then natural disasters are perhaps your worst fear.
    – Rick James
    Jul 2, 2018 at 13:06

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