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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4755) HBase based block placement in DFS
Date Fri, 15 Mar 2013 23:44:14 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4755?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13603991#comment-13603991
] 

Enis Soztutar commented on HBASE-4755:
--------------------------------------

bq. Online serving cases in general will notice the increased latency of remote reads
On failover, we are paying at least for the following:
 - replaying the log. Historically this has been the biggest problem, but should be getting
better. And with Jeffrey's HBASE-7835, I would expect this becomes even better. We are not
doing local reads to the log files here, and Sanjay's proposal does not help in this case.
But the data is supposed to be one-two orders of magnitude smaller than in hfiles. 
 - filling up the block cache: I think this is the biggest cost we are paying for online serve.
HDFS improvements wont help us here.
 - remote reads to fill up the block cache: without HDFS-2576, we will get eventual local
reads. On failover, we can trigger all the blocks of all files to be copied local. I think
we have to quantify how eventual this would be, and what we are paying for. 
                
> HBase based block placement in DFS
> ----------------------------------
>
>                 Key: HBASE-4755
>                 URL: https://issues.apache.org/jira/browse/HBASE-4755
>             Project: HBase
>          Issue Type: New Feature
>    Affects Versions: 0.94.0
>            Reporter: Karthik Ranganathan
>            Assignee: Christopher Gist
>            Priority: Critical
>         Attachments: 4755-wip-1.patch, hbase-4755-notes.txt
>
>
> The feature as is only useful for HBase clusters that care about data locality on regionservers,
but this feature can also enable a lot of nice features down the road.
> The basic idea is as follows: instead of letting HDFS determine where to replicate data
(r=3) by place blocks on various regions, it is better to let HBase do so by providing hints
to HDFS through the DFS client. That way instead of replicating data at a blocks level, we
can replicate data at a per-region level (each region owned by a promary, a secondary and
a tertiary regionserver). This is better for 2 things:
> - Can make region failover faster on clusters which benefit from data affinity
> - On large clusters with random block placement policy, this helps reduce the probability
of data loss
> The algo is as follows:
> - Each region in META will have 3 columns which are the preferred regionservers for that
region (primary, secondary and tertiary)
> - Preferred assignment can be controlled by a config knob
> - Upon cluster start, HMaster will enter a mapping from each region to 3 regionservers
(random hash, could use current locality, etc)
> - The load balancer would assign out regions preferring region assignments to primary
over secondary over tertiary over any other node
> - Periodically (say weekly, configurable) the HMaster would run a locality checked and
make sure the map it has for region to regionservers is optimal.
> Down the road, this can be enhanced to control region placement in the following cases:
> - Mixed hardware SKU where some regionservers can hold fewer regions
> - Load balancing across tables where we dont want multiple regions of a table to get
assigned to the same regionservers
> - Multi-tenancy, where we can restrict the assignment of the regions of some table to
a subset of regionservers, so an abusive app cannot take down the whole HBase cluster.

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