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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-1846) Don't fill preallocated portion of edits log with 0x00
Date Tue, 19 Apr 2011 23:02:06 GMT
Don't fill preallocated portion of edits log with 0x00
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                 Key: HDFS-1846
                 URL: https://issues.apache.org/jira/browse/HDFS-1846
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: name-node
    Affects Versions: 0.23.0
            Reporter: Aaron T. Myers
            Assignee: Aaron T. Myers


HADOOP-2330 added a feature to preallocate space in the local file system for the NN transaction
log. That change seeks past the current end of the file and writes out some data, which on
most systems results in the intervening data in the file being filled with zeros. Most underlying
file systems have special handling for sparse files, and don't actually allocate blocks on
disk for blocks of a file which consist completely of 0x00.

I've seen cases in the wild where the volume an edits dir is on fills up, resulting in a partial
final transaction being written out to disk. If you examine the bytes of this (now corrupt)
edits file, you'll see the partial final transaction followed by a lot of zeros, suggesting
that the preallocation previously succeeded before the volume ran out of space. If we fill
the preallocated space with something other than zeros, we'd likely see the failure at preallocation
time, rather than transaction-writing time, and so cause the NN to crash earlier, without
a partial transaction being written out.

I also hypothesize that filling the preallocated space in the edits log with something other
than 0x00 will result in a performance improvement in NN throughput. I haven't tested this
yet, but I intend to as part of this JIRA.

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