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https://issues.apache.org/jira/browse/HBASE-20003?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16374750#comment-16374750
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Anoop Sam John commented on HBASE-20003:
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bq.Currently, replica region is only for read, not for write, do we consider to 
promote replica region to "primary" in case of primary down?
Yes the idea is that.
bq.Another point is region numbers. With 1 primary + 2 replicas, the cluster 
will have 3 * normal-user-regions.
Agree this can be a concerning factor. Specially when the memstore is 
replicated across the replicas.  We should try to have max optimized memstore 
segments (As per diff segment types in compacting memstore) to reduce this 
overhead.  Some thing to consider. 

> WALLess HBase on Persistent Memory
> ----------------------------------
>
>                 Key: HBASE-20003
>                 URL: https://issues.apache.org/jira/browse/HBASE-20003
>             Project: HBase
>          Issue Type: New Feature
>            Reporter: Anoop Sam John
>            Assignee: Anoop Sam John
>            Priority: Major
>
> This JIRA aims to make use of persistent memory (pmem) technologies in HBase. 
> One such usage is to make the Memstore to reside on pmem. Making a persistent 
> memstore would remove the need for WAL and paves way for a WALLess HBase. 
> The existing region replica feature could be used here and ensure the data 
> written to memstores are synchronously replicated to the replicas and ensure 
> strong consistency of the data. (pipeline model)
> Advantages :
> - Data Availability : Since the data across replicas are consistent 
> (synchronously written) our data is always 100 % available.
> - Lower MTTR : It becomes easier/faster to switch over to the replicas on a 
> primary region failure as there is no WAL replay involved. Building the 
> memstore map data also is much faster than reading the WAL and replaying the 
> WAL.
> - Possibility of bigger memstores : These pmems are designed to have more 
> memory than DRAMs so it would also enable us to have bigger sized memstores 
> which leads to lesser flushes/compaction IO. 
> - Removes the dependency of HDFS on the write path
> Initial PoC has been designed and developed. Testing is underway and we would 
> publish the PoC results along with the design doc sooner. The PoC doc will 
> talk about the design decisions, the libraries considered to work with these 
> pmem devices, pros and cons of those libraries and the performance results.
> Note : Next gen memory technologies using 3DXPoint gives persistent memory 
> feature. Such memory DIMMs are soon to appear in the market. The PoC is done 
> around Intel's ApachePass (AEP)



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