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Date: Tue, 25 Feb 2014 10:11:49 -0800
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 <CALte62zDSoaW7b+2Ly+PeRKHhdeZDyUNq3VPm3XQ_ym24aWMKA@mail.gmail.com>
Subject: Re: Need some information over WAL
From: Ted Yu <yuzhihong@gmail.com>
To: "user@hbase.apache.org" <user@hbase.apache.org>,
 Dhaval Shah <prince_mithibai@yahoo.co.in>
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bq. Hbase uses append operation in hdfs to store/log WAL.

Following append operation, hdfs sync operation would be called for
persistence.

FYI


On Tue, Feb 25, 2014 at 10:05 AM, Dhaval Shah
<prince_mithibai@yahoo.co.in>wrote:

> Inline
>
> Regards,
> Dhaval
>
>
> ________________________________
>  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
>
> Thanks...

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