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HDFS >> mail # user >> Sizing help


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Re: Sizing help
3x replication has two effects.  One is reliability.  This is probably more
important in large clusters than small.

Another important effect is data locality during map-reduce.  Having 3x
replication allows mappers to have almost all invocations read from local
disk.  2x replication compromises this.  Even where you don't have local
data, the bandwidth available to read from 3x replicated data is 1.5x the
bandwidth available for 2x replication.

To get a rough feel for how reliable you should consider a cluster, you can
do a pretty simple computation.  If you have 12 x 2T on a single machine
and you lose that machine, the remaining copies of that data must be
replicated before another disk fails.  With HDFS and block-level
replication, the remaining copies will be spread across the entire cluster
to any disk failure is reasonably like to cause data loss.  For a 1000 node
cluster with 12000 disks, it is conservative to estimate a disk failure on
average every 2 hours.  Each node will have replicate about 12GB of data
which will take about 500 seconds or about 9 or 10 minutes if you only use
25% of your network for re-replication.  The probability of a disk failure
 during a 10 minute period is 1-exp(-10/120) = 8%.  This means that roughly
1 in 12 full machine failures might cause data loss.   You can pick
whatever you like for the rate at which nodes die, but I don't think that
this is acceptable.

My numbers for disk failures are purposely somewhat pessimistic.  If you
change the MTBF for disks to 10 years instead of 3 years, then the
probability of data loss after a machine failure drops, but only to about
2.5%.

Now, I would be the first to say that these numbers feel too high, but I
also would rather not experience enough data loss events to have a reliable
gut feel for how often they should occur.

My feeling is that 2x is fine for data you can reconstruct and which you
don't need to read really fast, but not good enough for data whose loss
will get you fired.

On Mon, Nov 7, 2011 at 7:34 PM, Rita <[EMAIL PROTECTED]> wrote:

> I have been running with 2x replication on a 500tb cluster. No issues
> whatsoever. 3x is for super paranoid.
>
>
> On Mon, Nov 7, 2011 at 5:06 PM, Ted Dunning <[EMAIL PROTECTED]> wrote:
>
>> Depending on which distribution and what your data center power limits
>> are you may save a lot of money by going with machines that have 12 x 2 or
>> 3 tb drives.  With suitable engineering margins and 3 x replication you can
>> have 5 tb net data per node and 20 nodes per rack.  If you want to go all
>> cowboy with 2x replication and little space to spare then you can double
>> that density.
>>
>> On Monday, November 7, 2011, Rita <[EMAIL PROTECTED]> wrote:
>> > For a 1PB installation you would need close to 170 servers with 12 TB
>> disk pack installed on them (with replication factor of 2). Thats a
>> conservative estimate
>> > CPUs: 4 cores with 16gb of memory
>> >
>> > Namenode: 4 core with 32gb of memory should be ok.
>> >
>> >
>> > On Fri, Oct 21, 2011 at 5:40 PM, Steve Ed <[EMAIL PROTECTED]> wrote:
>> >>
>> >> I am a newbie to Hadoop and trying to understand how to Size a Hadoop
>> cluster.
>> >>
>> >>
>> >>
>> >> What are factors I should consider deciding the number of datanodes ?
>> >>
>> >> Datanode configuration ?  CPU, Memory
>> >>
>> >> Amount of memory required for namenode ?
>> >>
>> >>
>> >>
>> >> My client is looking at 1 PB of  usable data and will be running
>> analytics on TB size files using mapreduce.
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> Thanks
>> >>
>> >> ….. Steve
>> >>
>> >>
>> >
>> >
>> > --
>> > --- Get your facts first, then you can distort them as you please.--
>> >
>>
>
>
>
> --
> --- Get your facts first, then you can distort them as you please.--
>
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB