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From Jim Kellerman <...@powerset.com>
Subject RE: HBase question on HRegions server
Date Thu, 01 Nov 2007 17:06:50 GMT
> -----Original Message-----
> From: Bin YANG [mailto:yangbinisme82@gmail.com]
> Sent: Thursday, November 01, 2007 3:06 AM
> To: hadoop-user@lucene.apache.org
> Subject: HBase question on HRegions server
> Hi,
> I am confused with some thing in HBase.
> 1. All data is stored in HDFS. Data is served to clients by
> HRegionServers. Is it allowed that the tablet T is on machine
> A, and served by a HRegionServers running on machine B?

Yes, it is possible. Depending how how many replicas of the data
there are in HDFS, it is possible that the data is on machines A,
B, C and the region server is running on machine D.

In the future, we will be investigating how to assign regions to
a region server based on where the data is located.

> What information does the META table maintain?
> The map from T to the physical address in machine A, or the
> map from T to which machine serves it, for example, machine B?

There are three pieces of data stored in the ROOT and META table:
1. The HRegionInfo object that describes the region. It includes
   the startKey, endKey, regionId, regionName and the HTableDescriptor
2. The host:port of the region server currently serving the region
3. A sequence number so that we can tell if the host:port is a
   current region assignment or if it is a stale assignment

> 2. Similar to Bigtable paper, what does the tablet location(section
> 5.1) stand for? Is it the map from the tablet id to physical
> address, or the map from the tablet to which machine serves it?

I don't know exactly what Google stores in their meta table. What
HBase stores is the data above. From it we can contact a region
server directly and the region server can locate the region's files
in HDFS.

> thanks
> --
> Bin YANG
> Department of Computer Science and Engineering Fudan
> University Shanghai, P. R. China
> EMail: yangbinisme82@gmail.com

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