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From see1230 <see1...@163.com>
Subject Re:Re: disk used percentage is not symmetric on datanodes (balancer)
Date Mon, 25 Mar 2013 03:33:11 GMT
if  the balancer is not  running ,or with a low bandwith and slow reaction, i think  there
may have a signatual unsymmetric between datanodes .

At 2013-03-25 04:37:05,"Jamal B" <jm15119b@gmail.com> wrote:

Then I think the only way around this would be to decommission  1 at a time, the smaller nodes,
and ensure that the blocks are moved to the larger nodes.  And once complete, bring back in
the smaller nodes, but maybe only after you tweak the rack topology to match your disk layout
more than network layout to compensate for the unbalanced nodes.  

Just my 2 cents

On Sun, Mar 24, 2013 at 4:31 PM, Tapas Sarangi <tapas.sarangi@gmail.com> wrote:

Thanks. We have a 1-1 configuration of drives and folder in all the datanodes.


On Mar 24, 2013, at 3:29 PM, Jamal B <jm15119b@gmail.com> wrote:

On both types of nodes, what is your dfs.data.dir set to? Does it specify multiple folders
on the same set's of drives or is it 1-1 between folder and drive?  If it's set to multiple
folders on the same drives, it is probably multiplying the amount of "available capacity"
incorrectly in that it assumes a 1-1 relationship between folder and total capacity of the

On Sun, Mar 24, 2013 at 3:01 PM, Tapas Sarangi <tapas.sarangi@gmail.com> wrote:

Yes, thanks for pointing, but I already know that it is completing the balancing when exiting
otherwise it shouldn't exit. 
Your answer doesn't solve the problem I mentioned earlier in my message. 'hdfs' is stalling
and hadoop is not writing unless space is cleared up from the cluster even though "df" shows
the cluster has about 500 TB of free space. 


On Mar 24, 2013, at 1:54 PM, Balaji Narayanan (பாலாஜி நாராயணன்)
<balaji@balajin.net> wrote:

 -setBalancerBandwidth <bandwidth in bytes per second>

So the value is bytes per second. If it is running and exiting,it means it has completed the

On 24 March 2013 11:32, Tapas Sarangi <tapas.sarangi@gmail.com> wrote:

Yes, we are running balancer, though a balancer process runs for almost a day or more before
exiting and starting over.
Current dfs.balance.bandwidthPerSec value is set to 2x10^9. I assume that's bytes so about
2 GigaByte/sec. Shouldn't that be reasonable ? If it is in Bits then we have a problem.
What's the unit for "dfs.balance.bandwidthPerSec" ?


On Mar 24, 2013, at 1:23 PM, Balaji Narayanan (பாலாஜி நாராயணன்)
<lists@balajin.net> wrote:

Are you running balancer? If balancer is running and if it is slow, try increasing the balancer

On 24 March 2013 09:21, Tapas Sarangi <tapas.sarangi@gmail.com> wrote:

Thanks for the follow up. I don't know whether attachment will pass through this mailing list,
but I am attaching a pdf that contains the usage of all live nodes.

All nodes starting with letter "g" are the ones with smaller storage space where as nodes
starting with letter "s" have larger storage space. As you will see, most of the "gXX" nodes
are completely full whereas "sXX" nodes have a lot of unused space. 

Recently, we are facing crisis frequently as 'hdfs' goes into a mode where it is not able
to write any further even though the total space available in the cluster is about 500 TB.
We believe this has something to do with the way it is balancing the nodes, but don't understand
the problem yet. May be the attached PDF will help some of you (experts) to see what is going
wrong here...


Balancer know about topology,but when calculate balancing it operates only with nodes not
with racks.
You can see how it work in Balancer.java in  BalancerDatanode about string 509.

I was wrong about 350Tb,35Tb it calculates in such way :

For example:

avgutil=cluster_dfsused/cluster_capacity*100=57.14% used cluster capacity
Then we know avg node utilization (node_dfsused/node_capacity*100) .Balancer think that all
good if  avgutil +10>node_utilizazation>=avgutil-10.

Ideal case that all node used avgutl of capacity.but for 12TB node its only 6.5Tb and for
72Tb its about 40Tb.

Balancer cant help you.

Show me http://namenode.rambler.ru:50070/dfsnodelist.jsp?whatNodes=LIVE if you can.


In ideal case with replication factor 2 ,with two nodes 12Tb and 72Tb you will be able to
have only 12Tb replication data.

Yes, this is true for exactly two nodes in the cluster with 12 TB and 72 TB, but not true
for more than two nodes in the cluster.

Best way,on my opinion,it is using multiple racks.Nodes in rack must be with identical capacity.Racks
must be identical capacity.
For example:

rack1: 1 node with 72Tb
rack2: 6 nodes with 12Tb
rack3: 3 nodes with 24Tb

It helps with balancing,because dublicated  block must be another rack.

The same question I asked earlier in this message, does multiple racks with default threshold
for the balancer minimizes the difference between racks ?

Why did you select hdfs?May be lustre,cephfs and other is better choise. 

It wasn't my decision, and I probably can't change it now. I am new to this cluster and trying
to understand few issues. I will explore other options as you mentioned.



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