Gang Xie created HDFS-13378:
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Summary: Add the option to set the du reserve by percent on
datanode
Key: HDFS-13378
URL: https://issues.apache.org/jira/browse/HDFS-13378
Project: Hadoop HDFS
Issue Type: Improvement
Components: datanode
Affects Versions: 3.0.1
Reporter: Gang Xie
Recently, we could event we reserved some space(dfs.datanode.du.reserved), this
disk still gets full, in turn, the performance is downgraded. After some test,
we found that we need consider the space used by ext4 (we use ext4 as local
fs). I.E, if the datanode reports there is 8T space available, you could not
write 8T data actually. About %4-%5 space is used for the ext4 metadata. The
following is the metrics of ext4 meta usage in disks with different capacity
(GB, reserved block for root is set to 1%, 120MB blocks):
|*FileSystem Capacity*|*Ext4 Data*|*Ex4 metadata*|*Metadata Ratio*|
|3666|3629|37|0.0101|
|7392|7318|74|0.0100|
|733|725|8|0.0109|
So, it seems that the ext4 most likely has the same meta data ratio with the
same about data and dir tree structure.
On the other hand, in our data center, there are several disk types with
different capacity. It's inefficient to set a absolute value for each datanode
according to the disk capacity. To us, it makes sense to set the reservation
with a ratio since we always use ext4 as the local fs. I think it's the same
case for most of other data center.
Our idea is to leave the default behavior unchanged. User could still use
dfs.datanode.du.reserved to set the reservation, but switch to use a ratio if
some other specified param is set.
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