Appends to already-finalized blocks can rename across volumes
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Key: HDFS-822
URL: https://issues.apache.org/jira/browse/HDFS-822
Project: Hadoop HDFS
Issue Type: Improvement
Components: data-node
Affects Versions: 0.21.0, 0.22.0
Reporter: Todd Lipcon
This is a performance thing. As I understand the code in FSDataset.append, if
the block is already finalized, it needs to move it into the RBW directory so
it can go back into a "being written" state. This is done using
volumes.getNextVolume without preference to the volume that the block currently
exists on. It seems to me that this could cause a lot of slow cross-volume
copies on applications that periodically append/close/append/close a file.
Instead, getNextVolume could provide an alternate form that gives preference to
a particular volume, so the rename stays on the same disk.
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