Under-replicated blocks will be prioritized, queued and replicated according to the logic in UnderReplicatedBlocks.java "Keep prioritized queues of under replicated blocks.
Blocks have replication priority, with priority QUEUE_HIGHEST_PRIORITY indicating the highest priority.
Having a prioritised queues allows the BlockManager to select which blocks to replicate first -it tries to give priority to data that is most at risk or considered most valuable." This method priorities under-replicated blocks: /** Return the priority of a block
* @param block a under replicated block
* @param curReplicas current number of replicas of the block
* @param expectedReplicas expected number of replicas of the block
* @return the priority for the blocks, between 0 and ({@link #LEVEL}-1)
*/
private int getPriority(Block block,
int curReplicas,
int decommissionedReplicas,
int expectedReplicas) {
assert curReplicas >= 0 : "Negative replicas!";
if (curReplicas >= expectedReplicas) {
// Block has enough copies, but not enough racks
return QUEUE_REPLICAS_BADLY_DISTRIBUTED;
} else if (curReplicas == 0) {
// If there are zero non-decommissioned replicas but there are
// some decommissioned replicas, then assign them highest priority
if (decommissionedReplicas > 0) {
return QUEUE_HIGHEST_PRIORITY;
}
//all we have are corrupt blocks
return QUEUE_WITH_CORRUPT_BLOCKS;
} else if (curReplicas == 1) {
//only on replica -risk of loss
// highest priority
return QUEUE_HIGHEST_PRIORITY;
} else if ((curReplicas * 3) < expectedReplicas) {
//there is less than a third as many blocks as requested;
//this is considered very under-replicated
return QUEUE_VERY_UNDER_REPLICATED;
} else {
//add to the normal queue for under replicated blocks
return QUEUE_UNDER_REPLICATED;
}
}
Queues are order as follows: QUEUE_HIGHEST_PRIORITY: the blocks that must be replicated first. That is blocks with only one copy, or blocks with zero live copies but a copy in a node being decommissioned. These blocks are at risk of loss if the disk or server on which they remain fails. QUEUE_VERY_UNDER_REPLICATED: blocks that are very under-replicated compared to their expected values. Currently that means the ratio of the ratio of actual:expected means that there is less than 1:3. These blocks may not be at risk, but they are clearly considered "important". QUEUE_UNDER_REPLICATED: blocks that are also under replicated, and the ratio of actual:expected is good enough that they do not need to go into the QUEUE_VERY_UNDER_REPLICATED queue. QUEUE_REPLICAS_BADLY_DISTRIBUTED: there are as least as many copies of a block as required, but the blocks are not adequately distributed. Loss of a rack/switch could take all copies off-line. QUEUE_WITH_CORRUPT_BLOCKS: This is for blocks that are corrupt and for which there are no-non-corrupt copies (currently) available. The policy here is to keep those corrupt blocks replicated, but give blocks that are not corrupt higher priority.
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