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From "Eli Collins (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1377) Quota bug for partial blocks allows quotas to be violated
Date Sun, 05 Sep 2010 18:34:33 GMT
Quota bug for partial blocks allows quotas to be violated 

                 Key: HDFS-1377
                 URL: https://issues.apache.org/jira/browse/HDFS-1377
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: name-node
    Affects Versions: 0.21.0, 0.20.2, 0.20.1
            Reporter: Eli Collins
            Assignee: Eli Collins
            Priority: Blocker
             Fix For: 0.20.3, 0.21.1, 0.22.0

There's a bug in the quota code that causes them not to be respected when a file is not an
exact multiple of the block size. Here's an example:

$ hadoop fs -mkdir /test
$ hadoop dfsadmin -setSpaceQuota 384M /test
$ ls dir/ | wc -l   # dir contains 101 files
$ du -ms dir        # each is 3mb
304	dir
$ hadoop fs -put dir /test
$ hadoop fs -count -q /test
        none             inf       402653184      -550502400            2          101   
      317718528 hdfs://haus01.sf.cloudera.com:10020/test
$ hadoop fs -stat "%o %r" /test/dir/f30
134217728 3    # three 128mb blocks

INodeDirectoryWithQuota caches the number of bytes consumed by it's children in {{diskspace}}.
The quota adjustment code has a bug that causes {{diskspace}} to get updated incorrectly when
a file is not an exact multiple of the block size (the value ends up being negative). 

This causes the quota checking code to think that the files in the directory consumes less
space than they actually do, so the verifyQuota does not throw a QuotaExceededException even
when the directory is over quota. However the bug isn't visible to users because {{fs count
-q}} reports the numbers generated by INode#getContentSummary which adds up the sizes of the
blocks rather than use the cached INodeDirectoryWithQuota#diskspace value.

In FSDirectory#addBlock the disk space consumed is set conservatively to the full block size
* the number of replicas:

updateCount(inodes, inodes.length-1, 0,
    fileNode.getPreferredBlockSize()*fileNode.getReplication(), true);

In FSNameSystem#addStoredBlock we adjust for this conservative estimate by subtracting out
the difference between the conservative estimate and what the number of bytes actually stored

//Updated space consumed if required.
INodeFile file = (storedBlock != null) ? storedBlock.getINode() : null;
long diff = (file == null) ? 0 :
    (file.getPreferredBlockSize() - storedBlock.getNumBytes());

if (diff > 0 && file.isUnderConstruction() &&
    cursize < storedBlock.getNumBytes()) {
    dir.updateSpaceConsumed(path, 0, -diff*file.getReplication());

We do the same in FSDirectory#replaceNode when completing the file, but at a file granularity
(I believe the intent here is to correct for the cases when there's a failure replicating
blocks and recovery). Since oldnode is under construction INodeFile#diskspaceConsumed will
use the preferred block size  (vs of Block#getNumBytes used by newnode) so we will again subtract
out the difference between the full block size and what the number of bytes actually stored

long dsOld = oldnode.diskspaceConsumed();
//check if disk space needs to be updated.
long dsNew = 0;
if (updateDiskspace && (dsNew = newnode.diskspaceConsumed()) != dsOld) {
  try {
    updateSpaceConsumed(path, 0, dsNew-dsOld);

So in the above example we started with diskspace at 384mb (3 * 128mb) and then we subtract
375mb (to reflect only 9mb raw was actually used) twice so for each file the diskspace for
the directory is - 366mb (384mb minus 2 * 375mb). Which is why the quota gets negative and
yet we can still write more files.

So a directory with lots of single block files (if you have multiple blocks on the final partial
block ends up subtracting from the diskspace used) ends up having a quota that's way off.

I think the fix is to in FSDirectory#replaceNode not have the diskspaceConsumed calculations
differ when the old and new INode have the same blocks. I'll work on a patch which also adds
a quota test for blocks that are not multiples of the block size and warns in INodeDirectory#computeContentSummary
if the computed size does not reflect the cached value.

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