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From Dhaval Shah <prince_mithi...@yahoo.co.in>
Subject Re: Need some information over WAL
Date Tue, 25 Feb 2014 18:05:19 GMT

 From: Upendra Yadav <upendra1024@gmail.com>
To: user@hbase.apache.org 
Sent: Tuesday, 25 February 2014 1:00 PM
Subject: Need some information over WAL

I have also doubt over WAL(write ahead log).

In hdfs we can write a new file or we can append to old file.

Is that correct :
1. WAL only logs operations not its data... just like disk journaling(ext4)
- No WAL is a log of all new data, not just the operations

2. In case of Region Server failure... WAL replay will depends on Client to
get each operation's data that yet not committed/flushed to hdfs.
- No client will not generally know of a region server failure. It gets the data from the
WAL and replays it. The client may not even exist when a region server crashes

3. Hbase uses append operation in hdfs to store/log WAL.
- Yes

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