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Raghu Angadi commented on HADOOP1463:

'df' is very cheap so calculating at every heart beat not bad either.
> Shall we enforce that only one of them is nonzero?
Not necessary depending on calculation of 'remaining space' 'reserved space' in your equations
above. How are those calculated?
According requirements of this jira, this is what I understand the calculation of available
space for datanode is:
{noformat}
remainig space for Datanode =
Min( (dfs.datanode.du.pct * Total_Capacity  cur_space_used_by_Datanode),
cur_disk_available)  dfs.datanode.du.reserved
{noformat}
cur_disk_available (and total capacity) comes from 'df' and cur_space_used_by_Datanode is
based on 'du'.
The descriptions of these config variables should probably change.
> dfs should report total size of all the space that dfs is using
> 
>
> Key: HADOOP1463
> URL: https://issues.apache.org/jira/browse/HADOOP1463
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Affects Versions: 0.12.3
> Reporter: Hairong Kuang
> Assignee: Hairong Kuang
> Fix For: 0.14.0
>
>
> Currently namenode reports two statistics back to the client:
> 1. The total capacity of dfs. This is a sum of all datanode's capacities, each of which
is calculated by datanode summing all data directories disk space.
> 2. The total remaining space of dfs. This is a sum of all datanodes's remaining space.
Each datanode's remaining space is calculated by using the following formula: remaining space
= unused space  capacity*unusableDiskPercentage  reserved space. So the remaining space
shows how much space that the dfs can still use, but it does not show the size of unused space.
> Each dfs client caculates the total dfs used space by substracting remaining space from
the total capacity. So the used space does not accurately shows the space that dfs is using.
However it is a very important number that dfs should provide.

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