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From Emmanuel Lécharny <elecha...@gmail.com>
Subject Re: [Txn Layer] WAL flush questions
Date Mon, 19 Mar 2012 17:46:07 GMT
Le 3/19/12 6:27 PM, Selcuk AYA a écrit :
> On Mon, Mar 19, 2012 at 10:26 AM, Selcuk AYA<ayaselcuk@gmail.com>  wrote:
>> On Mon, Mar 19, 2012 at 9:24 AM, Emmanuel Lécharny<elecharny@gmail.com>  wrote:
>>> Hi,
>>>
>>> I have a few questions about the handling of the log buffer.
>>>
>>> When we can't write anymore data in the buffer, because it's full, we try to
>>> flush the buffer on disk. What happens then is :
>>> - if there is enough room remaining in the buffer, we write a skip record
>>> (with a -1 length) : is it necessary ? (we then rewind the buffer)
>>> - otherwise, we rewind the buffer
>>>
>>> In any case, we increment the writeAheadRewindCount : what for ?
> as far as I can remember, writeAheadRewindCount was to avoid
> overwriting non flushed log records when in memory circular buffer
> wraps. IF this answer is not good enough, I can take a look more
> closely later.
Give me a bit of time to check the code again if you don't remember the 
exact usage. I was wondering if it was more or less a protection against 
a second flush thread being executed (for instance by an explicit call) 
while a flush is called when the buffer is full.

AFAICT, the flush thread check the LSN to know when to stop flushing 
records.
Not sure though.

-- 
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com


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