ramkrishna.s.vasudevan commented on HBASE-20003:

bq.Can't assume in a failure the primary (or even any replica at all) will 
Are you talking about the case where all the replicas including primary are 
down (total crash) and how to retrieve the data? As in HDFS case we can say min 
replication is 3 but asynchronously we can keep replicating to other DNs? 
Ya we can have that kind of mechanism here too. Try to replicate to other 
replicas which are above min replica asynchronously. We cannot guarantee all 
the data but atleast recover most of the inmemory data. How ever all the 
flushed data is any way in HDFS only so we don miss that. 

> 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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