Hello,
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again.
I'm comparing HBase and Cassandra, which I think are the most promising
distributed key-value stores, to determine which one to choose for the
future OLTP and data analysis.
I found the following benchmark report by Yahoo! Research which evalutes
HBase, Cassandra, PNUTS, and sharded MySQL.
http://wiki.apache.org/hadoop/Hbase/DesignOverview
The above report refers to HBase 0.20.3.
Reading this and HBase's documentation, two questions about load balancing
and replication have risen. Could anyone give me any information to help
solve these questions?
[Q1] Load balancing
Does HBase move regions to a newly added region server (logically, not
physically on storage) immediately? If not immediately, what timing?
On what criteria does the master unassign and assign regions among region
servers? CPU load, read/write request rates, or just the number of regions
the region servers are handling?
According the HBase design overview on the page below, the master monitors
the load of each region server and moves regions.
http://wiki.apache.org/hadoop/Hbase/DesignOverview
The related part is the following:
----------------------------------------
HMaster duties:
Assigning/unassigning regions to/from HRegionServers (unassigning is for
load balance)
Monitor the health and load of each HRegionServer
...
If HMaster detects overloaded or low loaded H!RegionServer, it will unassign
(close) some regions from most loaded H!RegionServer. Unassigned regions
will be assigned to low loaded servers.
----------------------------------------
When I read the above, I thought that the master checks the load of region
servers periodically (once a few minutes or so) and performs load balancing.
And I thought that the master unassigns some regions from the existing
loaded region servers to a newly added one immediately when the new server
joins the cluster and contacts the master.
However, the benchmark report by Yahoo! Research describes as follows. This
says that HBase does not move regions until compaction, so I cannot get the
effect of adding new servers immediately even if I added the new server to
solve the overload problem.
What's the fact?
----------------------------------------
6.7 Elastic Speedup
As the figure shows, the read latency spikes initially after
the sixth server is added, before the latency stabilizes at a
value slightly lower than the latency for five servers. This result
indicates that HBase is able to shift read and write load
to the new server, resulting in lower latency. HBase does
not move existing data to the new server until compactions
occur2. The result is less latency variance compared to Cassandra
since there is no repartitioning process competing
for the disk. However, the new server is underutilized, since
existing data is served off the old servers.
...
2 It is possible to run the HDFS load balancer to force data
to the new servers, but this greatly disrupts HBase’s ability
to serve data partitions from the same servers on which they
are stored.
----------------------------------------
MauMau
