Minor Description
Afaik, HDFS does not provide an easy way to temporarily disable the replication of missing blocks.
If you would like to temporarily disable the replication, you would have to
- set dfs.namenode.replication.interval (The periodicity in seconds with which the namenode computes repliaction work for datanodes Default 3) to something very high. Disadvantage: you have to restart the NN
- go into the safe-mode. Disadvantage: all write operations will fail
We have the situation that we need to replace our top-of-rack switches for each rack. Replacing a switch should take around 30 minutes. Each rack has around 0.6 PB of data. We would like to avoid an expensive replication, since we know that we will put this rack online quickly. To avoid any downtime, or excessive network transfer, we think that temporarily disabling the replication could fit us.
The default block placement policy puts blocks into two racks, so when one rack temporarily goes offline, we still have an access to at least replica of each block. Of course, if we lose this replica, then we would have to wait until the rack goes back online. This is what the administrator should be aware of.
This feature could disable the replication
- globally - for a whole cluster
- partially - e.g. only for missing blocks that come from a specified set of DataNodes. So a file like "we_will_be_back_soon"
could be introduced, similar to include and exclude.