Re: questions on undo log

Xuekun,

On 08/02/2012, at 1:03 PM, Xuekun Hu wrote:

> Hi, sunny
> 
> On Wed, Feb 8, 2012 at 9:50 AM, Sunny Bains <[email protected]> wrote:
>> Xuekun,
>> 
>> On 08/02/2012, at 12:02 PM, Xuekun Hu wrote:
>> 
>>> Hi, Libing
>>> 
>>> Many thanks. From the file page types definition, I found
>>> FIL_PAGE_UNDO_LOG which should represent the undo log page in buffer
>>> pool. So it also means every change to undo log page in buffer pool
>>> also follows the page flushing mechanism of buffer pool (writing to
>>> disk in background, writing to doublewrite buffer, writing the changes
>>> to transaction log etc). My understanding is right?
>>> 
>>> However I also think it is not very accurate about "Innodb buffer pool
>>> is for all things in table spaces" like you said. :-) I think it
>>> should exclude the data dictionary which has own memory cache and
>>> doublewrite buffer. :-)
>> 
>> Song Libing is correct.  The data dictionary cache is simply another view
>> of the data dictionary that is on disk. There is no separate buffer pool
>> for the dictionary cache and it doesn't have a separate double write buffer.
>> 
> 
> Sorry, I don't quite understand the meanings of "separate". You mean
> the dictionary cache is a part of buffer pool? Also the 2MB of
> doublewrite buffer in memory  is also a part of buffer pool?

There are many sub-systems that use the buffer pool as a backing store.  Heap
creation takes a flag that determines which backing store to use.  Using the buffer
pool memory is one such flag. By separate I meant that the data dictionary doesn't
use a special double write buffer.

Regards,
-sunny
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