[HDFS-17205] HdfsServerConstants.MIN_BLOCKS_FOR_WRITE should be configurable

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.4.0
• Fix Version/s: 3.4.0
• Component/s: hdfs
• Labels: pull-request-available
• Target Version/s: 3.4.0
• Hadoop Flags: Reviewed
• Assignee: Haiyang Hu
• Reporter: Haiyang Hu
• Created: 23/Sep/23 05:23
• Updated: 28/Jan/24 01:26
• Resolved: 06/Oct/23 07:09

Description

Current allocate new block , the NameNode will choose datanode and choose a good storage of given storage type from datanode, the specific calling code is DatanodeDescriptor#chooseStorage4Block, here will calculate the space required for write operations,
requiredSize = blockSize * HdfsServerConstants.MIN_BLOCKS_FOR_WRITE(default is 1).

public DatanodeStorageInfo chooseStorage4Block(StorageType t,
    long blockSize) {
  final long requiredSize =
      blockSize * HdfsServerConstants.MIN_BLOCKS_FOR_WRITE;
  final long scheduledSize = blockSize * getBlocksScheduled(t);
  long remaining = 0;
  DatanodeStorageInfo storage = null;
  for (DatanodeStorageInfo s : getStorageInfos()) {
    if (s.getState() == State.NORMAL && s.getStorageType() == t) {
      if (storage == null) {
        storage = s;
      }
      long r = s.getRemaining();
      if (r >= requiredSize) {
        remaining += r;
      }
    }
  }
  if (requiredSize > remaining - scheduledSize) {
    BlockPlacementPolicy.LOG.debug(
        "The node {} does not have enough {} space (required={},"
        + " scheduled={}, remaining={}).",
        this, t, requiredSize, scheduledSize, remaining);
    return null;
  }
  return storage;
}

But when multiple NameSpaces select the storage of the same datanode to write blocks at the same time.
In extreme cases, if there is only one block size left in the current storage, there will be a situation where there is not enough free space for the writer to write data.

log similar to the following appears:

The volume [file:/disk1/] with the available space (=21129618 B) is less than the block size (=268435456 B).  

In order to avoid this case, consider HdfsServerConstants.MIN_BLOCKS_FOR_WRITE should be configurable, and the parameters can be adjusted in larger clusters.

Issue Links

• links to: GitHub Pull Request #6112