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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2991) dfs.du.reserved not honored in 0.15/16 (regression from 0.14+patch for 2549)
Date Tue, 11 Mar 2008 06:00:47 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577314#action_12577314
] 

Hairong Kuang commented on HADOOP-2991:
---------------------------------------

I think we should discuss the meaning of all the terms before we discuss the changes. I see
that Pete and Joydeep kept on saying "off".

This is what we defined in hadoop-1463
dfs capacity = the total disk space that data directories are located. This does not mean
the total dfs usable space.
dfs used space = the space that dfs used
reserved space = the space reserved for non-dfs usage
remaining = the total free disk space available to dfs, which is equal to MIN(dfs capacity-reserved
space, disk available space)*du.pct. 

Block placement is based on the remaining space. So dfs should never use more than (dfs capacity-reserved
space) space.

Of course dfs capacity != dfs used space + remaining since disks are shared by dfs and non-dfs
applications. This is similar to disk capacity != used space + available space because disks
are shared by user applications and O.S.

> dfs.du.reserved not honored in 0.15/16 (regression from 0.14+patch for 2549)
> ----------------------------------------------------------------------------
>
>                 Key: HADOOP-2991
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2991
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.15.0, 0.15.1, 0.15.2, 0.15.3, 0.16.0
>            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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