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https://issues.apache.org/jira/browse/HDFS-7645?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14286576#comment-14286576
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Colin Patrick McCabe commented on HDFS-7645:
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bq. DN layout changes will be rare for minor/point releases. I am wary of
eliminating trash without some numbers showing hard link performance with
millions of blocks is on par with trash. Even a few seconds per DN adds up to
many hours/days when upgrading thousands of DNs sequentially. Once we fix this
issue raised by Nathan the overhead of trash as compared to regular startup is
nil.
Yeah. Startup time during an upgrade is important. Our numbers for creating
the "previous" directory in HDFS-6482 were about 1 second per 100,000 blocks.
We also parallelized the hard link process across all volumes. So I would
expect it to be very quick for the average DN, which has about 200k-400k blocks
split across 10 storage directories.
Anyway, I don't feel strongly about this... if we can make "trash" work, then
so be it. It sounds like the fix is not that difficult.
> Rolling upgrade is restoring blocks from trash multiple times
> -------------------------------------------------------------
>
> Key: HDFS-7645
> URL: https://issues.apache.org/jira/browse/HDFS-7645
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Nathan Roberts
>
> When performing an HDFS rolling upgrade, the trash directory is getting
> restored twice when under normal circumstances it shouldn't need to be
> restored at all. iiuc, the only time these blocks should be restored is if we
> need to rollback a rolling upgrade.
> On a busy cluster, this can cause significant and unnecessary block churn
> both on the datanodes, and more importantly in the namenode.
> The two times this happens are:
> 1) restart of DN onto new software
> {code}
> private void doTransition(DataNode datanode, StorageDirectory sd,
> NamespaceInfo nsInfo, StartupOption startOpt) throws IOException {
> if (startOpt == StartupOption.ROLLBACK && sd.getPreviousDir().exists()) {
> Preconditions.checkState(!getTrashRootDir(sd).exists(),
> sd.getPreviousDir() + " and " + getTrashRootDir(sd) + " should not
> " +
> " both be present.");
> doRollback(sd, nsInfo); // rollback if applicable
> } else {
> // Restore all the files in the trash. The restored files are retained
> // during rolling upgrade rollback. They are deleted during rolling
> // upgrade downgrade.
> int restored = restoreBlockFilesFromTrash(getTrashRootDir(sd));
> LOG.info("Restored " + restored + " block files from trash.");
> }
> {code}
> 2) When heartbeat response no longer indicates a rollingupgrade is in progress
> {code}
> /**
> * Signal the current rolling upgrade status as indicated by the NN.
> * @param inProgress true if a rolling upgrade is in progress
> */
> void signalRollingUpgrade(boolean inProgress) throws IOException {
> String bpid = getBlockPoolId();
> if (inProgress) {
> dn.getFSDataset().enableTrash(bpid);
> dn.getFSDataset().setRollingUpgradeMarker(bpid);
> } else {
> dn.getFSDataset().restoreTrash(bpid);
> dn.getFSDataset().clearRollingUpgradeMarker(bpid);
> }
> }
> {code}
> HDFS-6800 and HDFS-6981 were modifying this behavior making it not completely
> clear whether this is somehow intentional.
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