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From Joey Echeverria <j...@cloudera.com>
Subject Re: NameNode heapsize
Date Fri, 10 Jun 2011 20:16:29 GMT
Each "object" (file, directory, and block) uses about 150 bytes of
memory. If you lower the number of files by having larger ones, you
save a modest amount of memory, depending on how many blocks your
existing files use. The real savings comes from having larger files
and a larger block size. Lets say you start by having some number of
files where each file fits in a single block (64 MB). If you double
the average block usage to two (<=128MB files) by lowering your number
of files in half, you'll save up to 1/3 of your NN memory. If you also
double your block size, you'll cut your usage by another 1/3. This
becomes very significant very quickly.


On Fri, Jun 10, 2011 at 12:36 PM, Anh Nguyen <anguyen@redhat.com> wrote:
> On 06/10/2011 04:57 AM, Joey Echeverria wrote:
>> Hi On,
>> The namenode stores the full filesystem image in memory. Looking at
>> your stats, you have ~30 million files/directories and ~47 million
>> blocks. That means that on average, each of your files is only ~1.4
>> blocks in size.  One way to lower the pressure on the namenode would
>> be to store fewer, larger files. If you're able to concatenate files
>> and still parse them, great. Otherwise, Hadoop provides a couple of
>> container file formats that might help.
>> SequenceFiles are Hadoop specific binary files that store key/value
>> pairs. If your data fits that model, you can convert the data into
>> SequenceFiles when you write it to HDFS, including data from multiple
>> input files in a single SequenceFile. Here is a simple example of
>> using the SequenceFile API:
>> http://programmer-land.blogspot.com/2009/04/hadoop-sequence-files.html
>> Another options are Hadoop Archive files (HARs). A HAR file lets you
>> combine multiple smaller files into a virtual filesystem. Here are
>> some links with details on HARs:
>> http://developer.yahoo.com/blogs/hadoop/posts/2010/07/hadoop_archive_file_compaction/
>> http://hadoop.apache.org/mapreduce/docs/current/hadoop_archives.html
>> If you're able to use any of these techniques to grow your average
>> file size, then you can also save memory by increasing the block size.
>> The default block size is 64MB, most clusters I've been exposed to run
>> at 128MB.
>> -Joey
> Hi Joey,
> The explanation is really helpful as I've been looking for way to size the
> NN heap.
> What I am still not clear though is why changing the average file size would
> save NN heap usage.
> Since it contains the entire FS image, the sum of space would be the same
> regardless of file size.
> Or is it not the case because with larger file size there would be less
> meta-data to maintain.
> Thanks in advance for your clarification, particularly for advice on sizing
> the heap.
> Anh-
>> On Fri, Jun 10, 2011 at 7:45 AM, siuon@ugcv.com<siuon@ugcv.com>  wrote:
>>> Dear all,
>>> I'm looking for ways to improve the namenode heap size usage of a
>>> 800-node
>>> 10PB testing Hadoop cluster that stores around 30 million files.
>>> Here's some info:
>>> 1 x namenode:     32GB RAM, 24GB heap size
>>> 800 x datanode:   8GB RAM, 13TB hdd
>>> 33050825 files and directories, 47708724 blocks = 80759549 total. Heap
>>> Size
>>> is 22.93 GB / 22.93 GB (100%)
>>>  From the cluster summary report, it seems the heap size usage is always
>>> full
>>> but couldn't drop, do you guys know of any ways to reduce it ? So far I
>>> don't see any namenode OOM errors so it looks memory assigned for the
>>> namenode process is (just) enough. But i'm curious which factors would
>>> account for the full use of heap size ?
>>> Regards,
>>> On

Joseph Echeverria
Cloudera, Inc.

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