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From "Marcelo Valle (BLOOMBERG/ LONDON)" <mvallemil...@bloomberg.net>
Subject Re: write availability
Date Tue, 07 Apr 2015 16:04:17 GMT
Wellington, 

I might be misinterpreting this: http://stackoverflow.com/questions/13741946/role-of-datanode-regionserver-in-hbase-hadoop-integration

But aren't HBase region servers and HDFS datanodes always in the same server? With a replication
factor of 3, what happens if all 3 datanodes hosting that information go down and one of them
come back, but with the disk intact? Considering from the time they went down to the time
it went back HBase received new writes that would go to the same data node...


From: user@hbase.apache.org 
Subject: Re: write availability

The data is stored on files on hdfs. If a RS goes down, the master knows which regions were
on that RS and which hdfs files contain data for these regions, so it will just assign the
regions to others RS, and these others RS will have access to the regions data because it's
stored on HDFS. The RS does not "own" the disk, this is HDFS job, so the recovery on this
case is transparent.


On 7 Apr 2015, at 16:51, Marcelo Valle (BLOOMBERG/ LONDON) <mvallemilita@bloomberg.net>
wrote:

> So if a RS goes down, it's assumed you lost the data on it, right?
> HBase has replications on HDFS, so if a RS goes down it doesn't mean I lost all the data,
as I could have the replicas yet... But what happens if all RS hosting a specific region goes
down? 
> What if one RS from this one comes back again, but with the disk intact, with all the
data it had before crashing?
> 
> 
> From: user@hbase.apache.org 
> Subject: Re: write availability
> 
> When a RS goes down, the Master will try to assign the regions on the remaining RSes.
When the RS comes back, after a while, the Master balancer process will re-distribute regions
between RS, so the given RS will be hosting regions, but not necessarily the one it used to
host before it went down.
> 
> 
> On 7 Apr 2015, at 16:31, Marcelo Valle (BLOOMBERG/ LONDON) <mvallemilita@bloomberg.net>
wrote:
> 
>>> So if the cluster is up, then you can insert records in to HBase even though
you lost a RS that was handing a specific region. 
>> 
>> What happens when the RS goes down? Writes to that region will be written to another
region server? Another RS assumes the region "range" while the RS is down?
>> 
>> What happens when the RS that was down goes up again? 
>> 
>> 
>> From: user@hbase.apache.org 
>> Subject: Re: write availability
>> 
>> I don’t know if I would say that… 
>> 
>> I read Marcelo’s question of “if the cluster is up, even though a RS may be down,
can I still insert records in to HBase?”
>> 
>> So if the cluster is up, then you can insert records in to HBase even though you
lost a RS that was handing a specific region. 
>> 
>> But because he talked about syncing nodes… I could be misreading his initial question…

>> 
>>> On Apr 7, 2015, at 9:02 AM, Serega Sheypak <serega.sheypak@gmail.com> wrote:
>>> 
>>>> If I have an application that writes to a HBase cluster, can I count that
>>> the cluster will always available to receive writes?
>>> No, it's CP, not AP system.
>>>> so everything get in sync when the other nodes get up again
>>> There is no hinted backoff, It's not Cassandra.
>>> 
>>> 
>>> 
>>> 2015-04-07 14:48 GMT+02:00 Marcelo Valle (BLOOMBERG/ LONDON) <
>>> mvallemilita@bloomberg.net>:
>>> 
>>>> If I have an application that writes to a HBase cluster, can I count that
>>>> the cluster will always available to receive writes?
>>>> I might not be able to read if a region server which handles a range of
>>>> keys is down, but will I be able to keep writing to other nodes, so
>>>> everything get in sync when the other nodes get up again?
>>>> Or I might get no write availability for a while?
>> 
>> The opinions expressed here are mine, while they may reflect a cognitive thought,
that is purely accidental. 
>> Use at your own risk. 
>> Michael Segel
>> michael_segel (AT) hotmail.com
> 
> 


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