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From "Brock Noland (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-1846) Don't fill preallocated portion of edits log with 0x00
Date Mon, 25 Apr 2011 02:14:06 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13024706#comment-13024706
] 

Brock Noland commented on HDFS-1846:
------------------------------------

This is a major improvement to the existing preallocation scheme. If we decide the time to
preallocate is too costly, then I think I the existing scheme should be scrapped. Sparse files
are not preallocated. You can create a file which appears to be 500GB instantly by seeking
past the end of the file.

# rm -f test; time dd if=/dev/null of=test seek=999999999; ls -lh test
0+0 records in
0+0 records out
0 bytes (0 B) copied, 1.4e-05 seconds, 0.0 kB/s

real	0m0.002s
user	0m0.000s
sys	0m0.002s
-rw-r--r-- 1 root root 477G Apr 24 21:04 test


> Don't fill preallocated portion of edits log with 0x00
> ------------------------------------------------------
>
>                 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
>         Attachments: hdfs-1846-perf-analysis.0.patch, hdfs-1846.0.txt
>
>
> 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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