[jira] Commented: (HADOOP-158) dfs block ids sometimes collide

["Doug Cutting (JIRA)" <ji...@apache.org>]

    [ http://issues.apache.org/jira/browse/HADOOP-158?page=comments#action_12377273 ] 

Doug Cutting commented on HADOOP-158:
-------------------------------------

Using the formulas in:

http://en.wikipedia.org/wiki/Birthday_paradox#Generalisation

I think it is actually very unlikely that, with 64-bit block ids and a decent random number generator, we are actually seeing collisions.  It seems more likely that the symptoms ascribed to duplicate block id allocations are actually the result of other bugs.  Still, it would be more comfortable to not rely on random block id allocation long-term.

> dfs block ids sometimes collide
> -------------------------------
>
>          Key: HADOOP-158
>          URL: http://issues.apache.org/jira/browse/HADOOP-158
>      Project: Hadoop
>         Type: Bug

>   Components: dfs
>     Reporter: Doug Cutting
>     Assignee: Konstantin Shvachko
>      Fix For: 0.2

>
> 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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