Re: Uncommitted Data

agarwalp_at_eeism.com wrote:
> Hi,
>
> Following is the excerpt from an article on Transaction logs under
> Checkpoints Heading by Peter Gulutzan
> (http://www.dbazine.com/db2/db2-disarticles/gulutzan2)
>
> "Users need recoveries to work faster, so there are times when the DBMS
> will write absolutely everything. This enables it to flush all dirty
> data pages to the database too. Then the DBMS writes a log record
> marking that a "checkpoint" has been done. If a crash occurs later,
> recovery is easy because the DBMS only needs to roll forward with log
> records since the last checkpoint"
>
> This is somewhat confusing me. What i interpret is that when the DBMS
> writes dirty data records to the datafiles during a check point, it
> updqate the log file saying that a check point is done. So my question
> is why & what does it have to roll forward, even though i have the data
> on the physical datafile.

I thought it was pretty clear. It has to roll forwards things that were done since the beginning of the last completed checkpoint. Why does it have to roll that forward? Because otherwise the transactions after the last completed checkpoint but before the crash woud be lost.

Xho

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