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https://issues.apache.org/jira/browse/HADOOP-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577265#action_12577265
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Raghu Angadi commented on HADOOP-2991:
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from HADOOP-1463 :
> folks - the implementation does not agree with the semantics discussed in the
> jira (or the semantics in 0.14)
Joydeep, what is the exact discrepancy between the implementation and the new
semantics (an example would help)? Yes, 'capacity' is not accurate. But it
FSDataset.FSVolume.getAvailable() does take into account, note that it never
returns more than what is in 'available' column of 'df'.
I agree with the later part : it is an incompatible change and it changed the
meaning of 'reserved'. Users might need better guidance.
> 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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