Article ID: 123249, created on Oct 24, 2014, last review on Jan 23, 2016

  • Applies to:
  • Virtuozzo 6.0

Question

I have the minimum replicas parameter set to 1. The following warning can be observed in pstorage events:

# pstorage -c mycluster get-event | less
...
08-07-14 15:14:55  MDS WRN Minimum replicas set to 1, which means that cluster can be unable to survive loss of a single machine!
...

Why is the cluster unable to survive the loss of a single node with 2:1 replication settings?

Answer

In case some data (replicas/chunks) for a Virtual Machine or container was lost - it is considered as a cluster problem - the reliability was not provided. In case of 2:1 replication settings and one host failure the cluster itself will continue functioning, but a situation can arise that will result is some data loss for the VMs/CTs - thus the warning.

The success of the write operation is not returned to the client unless the minimum number of replicas is created (the success of the write operation in user's terms: you are modifying a file in a text editor, e.g vim, and when you try to save the changes you are either successful or not).

If 2:1 replication is configured, there is a possibility that:

  • the client does some changes
  • a single replica is created, customer gets write operation confirmation
  • the creation of the second replica starts
  • during the time when replication is in progress but not yet finished, the node with the single valid replica crashes - customer's data is lost.

If 3:2 replication is configured:

  • the client does some changes
  • customer gets write operation confirmation only once there are 2 replicas
  • so after that if a host crashes, there is still a valid replica for the data.

In case of 2:1 settings the host which crashed also matters - whether it is the node with only the client and the chunk server with the single replica or if it is only the server with the replica. In case only the server with the replica crashes (taking the single valid replica with it) and the client runs on another server - replica can be recovered from the memory of the client.

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