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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-2991) dfs.du.reserved not honored in 0.15/16 (regression from 0.14+patch for 2549)
Date Mon, 10 Mar 2008 23:50:49 GMT
dfs.du.reserved not honored in 0.15/16 (regression from 0.14+patch for 2549)
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                 Key: HADOOP-2991
                 URL: https://issues.apache.org/jira/browse/HADOOP-2991
             Project: Hadoop Core
          Issue Type: Bug
          Components: dfs
    Affects Versions: 0.16.0, 0.15.3, 0.15.2, 0.15.1, 0.15.0
            Reporter: Joydeep Sen Sarma
            Priority: Critical


changes for https://issues.apache.org/jira/browse/HADOOP-1463

have caused a regression. earlier:

- we could set dfs.du.reserve to 1G and be *sure* that 1G would not be used.

now this is no longer true. I am quoting Pete Wyckoff's example:

<example>
Let's look at an example. 100 GB disk and /usr using 45 GB and dfs using 50 GBs now

Df -kh shows:

Capacity = 100 GB
Available = 1 GB (remember ~4 GB chopped out for metadata and stuff)
Used = 95 GBs   

remaining = 100 GB - 50 GB - 1GB = 49 GB 

Min(remaining, available) = 1 GB

98% of which is usable for DFS apparently - 

So, we're at the limit, but are free to use 98% of the remaining 1GB.
</example>

this is broke. based on the discussion on 1463 - it seems like the notion of 'capacity' as
being the first field of 'df' is problematic. For example - here's what our df output looks
like:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             130G  123G   49M 100% /


as u can see - 'Size' is a misnomer - that much space is not available. Rather the actual
usable space is 123G+49M ~ 123G. (not entirely sure what the discrepancy is due to - but have
heard this may be due to space reserved for file system metadata). Because of this discrepancy
- we end up in a situation where file system is out of space.


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