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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1071) savenamespace should write the fsimage to all configured fs.name.dir in parallel
Date Tue, 22 Jun 2010 23:17:50 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1071?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12881444#action_12881444
] 

Konstantin Shvachko commented on HDFS-1071:
-------------------------------------------

> saveNamespace can only be done when in safemode, so we are only performing read operations
on the datastructure that is essentially read only

Yes you can saveNamespace only only in safe mode, but the data structures are not read only.
For example, {{getBlockLocations()}} updates {{accessTime}} for file inodes. This means that
different threads in your implementation may record different atimes of the same file, depending
on when they write it. This means that file system images will not be identical in different
directories. They have been so far.

> while the parent thread is holding a lock, right?

Are you relying on the parent thread lock? Could you please explain how that works?

In fact I thought you would implement something like this: one thread traverses the tree,
serializes image objects and puts them into a queue, where other writing threads pick them
up and write to disk in parallel. Then it is guaranteed that images are exactly the same.

> so it will take 1.5-2 minutes for both.

Have you actually tested it? Is it the same if both directories are on the same drive?

> savenamespace should write the fsimage to all configured fs.name.dir in parallel
> --------------------------------------------------------------------------------
>
>                 Key: HDFS-1071
>                 URL: https://issues.apache.org/jira/browse/HDFS-1071
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: dhruba borthakur
>            Assignee: Dmytro Molkov
>         Attachments: HDFS-1071.2.patch, HDFS-1071.3.patch, HDFS-1071.4.patch, HDFS-1071.5.patch,
HDFS-1071.patch
>
>
> If you have a large number of files in HDFS, the fsimage file is very big. When the namenode
restarts, it writes a copy of the fsimage to all directories configured in fs.name.dir. This
takes a long time, especially if there are many directories in fs.name.dir. Make the NN write
the fsimage to all these directories in parallel.

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