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https://issues.apache.org/jira/browse/HDFS-1070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13015701#comment-13015701
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Tsz Wo (Nicholas), SZE commented on HDFS-1070:
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Here is an orthogonal idea for reducing image size:
In NameNode, we have internal maps for usernames, groups to serial numbers (see
{{SerialNumberManager}}) in order to save memory in the NameNode. How about we
do the same for {{FSImage}}? I.e. write the maps in the beginning of
{{FSImage}} and then use the serial numbers in the {{INode}} entries.
Suppose the saving is 10 bytes per name, that is 20 bytes per {{INode}}. Then,
it is about 1.1 GB for a namespace with 60 million files/directories.
> Speedup NameNode image loading and saving by storing local file names
> ---------------------------------------------------------------------
>
> Key: HDFS-1070
> URL: https://issues.apache.org/jira/browse/HDFS-1070
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: Hairong Kuang
> Assignee: Hairong Kuang
> Attachments: trunkLocalNameImage.patch, trunkLocalNameImage1.patch,
> trunkLocalNameImage3.patch, trunkLocalNameImage4.patch,
> trunkLocalNameImage5.patch
>
>
> Currently each inode stores its full path in the fsimage. I'd propose to
> store the local name instead. In order for each inode to identify its parent,
> all inodes in a directory tree are stored in the image in in-order. This
> proposal also requires each directory stores the number of its children in
> image.
> This proposal would bring a few benefits as pointed below and therefore
> speedup the image loading and saving.
> # Remove the overhead of converting java-UTF8 encoded local name to
> string-represented full path then to UTF8 encoded full path when saving to an
> image and vice versa when loading the image.
> # Remove the overhead of traversing the full path when inserting the inode to
> its parent inode.
> # Reduce the number of temporary java objects during the process of image
> loading or saving and therefore reduce the GC overhead.
> # Reduce the size of an image.
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