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From Алексей Бабутин <zorlaxpokemon...@gmail.com>
Subject Re: disk used percentage is not symmetric on datanodes (balancer)
Date Fri, 22 Mar 2013 16:05:34 GMT
2013/3/20 Tapas Sarangi <tapas.sarangi@gmail.com>

> Thanks for your reply. Some follow up questions below :
>
> On Mar 20, 2013, at 5:35 AM, Алексей Бабутин <zorlaxpokemonych@gmail.com>
> wrote:
>
>
>
> dfs.balance.bandwidthPerSec in hdfs-site.xml.I think balancer cant help
> you,because it makes all the nodes equal.They can differ only on balancer
> threshold.Threshold =10 by default.It means,that nodes can differ up to
> 350Tb between each other in 3.5Pb cluster.If Threshold =1 up to 35Tb and so
> on.
>
>
> If we use multiple racks, let's assume we have 10 racks now and they are
> equally divided in size (350 TB each). With a default threshold of 10, any
> two nodes on a given rack will have a maximum difference of 35 TB, is this
> correct ? Also, does this mean the difference between any two racks will
> also go down to 35 TB ?
>

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:
cluster_capacity=3.5Pb
cluster_dfsused=2Pb

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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