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From "Sameer Paranjpye (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-158) dfs should allocate a random blockid range to a file, then assign ids sequentially to blocks in the file
Date Tue, 30 May 2006 21:26:32 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-158?page=comments#action_12413909 ] 

Sameer Paranjpye commented on HADOOP-158:
-----------------------------------------

It can be sequential. In that case, the namenode would need to determine the lowest unused
file-id at startup and start file-id assignments from that point. 

Even sequential allocation of file-ids should probably do the collision check because you
don't need a trillion files in the system before you wrap around, you only need a trillion
file creation events. If you're doing the collision check in both schemes the random file-id
assignment keeps things simpler.

The possibility of collision with sequential assignment of file-ids is very remote, but why
expose ourselves? I'm probably being paranoid so ignore me on this one if you want.





> dfs should allocate a random blockid range to a file, then assign ids sequentially to
blocks in the file
> --------------------------------------------------------------------------------------------------------
>
>          Key: HADOOP-158
>          URL: http://issues.apache.org/jira/browse/HADOOP-158
>      Project: Hadoop
>         Type: Bug

>   Components: dfs
>     Versions: 0.1.0
>     Reporter: Doug Cutting
>     Assignee: Konstantin Shvachko
>      Fix For: 0.4

>
> A random number generator is used to allocate block ids in dfs.  Sometimes a block id
is allocated that is already used in the filesystem, which causes filesystem corruption.
> A short-term fix for this is to simply check when allocating block ids whether any file
is already using the newly allocated id, and, if it is, generate another one.  There can still
be collisions in some rare conditions, but these are harder to fix and will wait, since this
simple fix will handle the vast majority of collisions.

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