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Allen Wittenauer commented on HADOOP-2991:
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Available is *not* trustable.  It doesn't always take into consideration quotas, and reserved space.  [Oh how many times I've heard users complain with the "df says there is still room but I can't write to my home dir" statement...]  Oh, and inode counts.  I completely forgot about that little gem.

Anyway, yes, I completely agree--this needs to be settable on a per dir basis.  That's actually something I've wanted for a while, since we store logs on the same dir as the data node.  I want more space available on the node with the logs than the others.  I might even have a JIRA on this somewhere.... ahh yes, here it is:

https://issues.apache.org/jira/browse/HADOOP-2150

> 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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