Re: interaction between LGWR and DBWR

Hi, Umar.

1.
LGWR and DBWR will write data simultaneously just in the case of a checkpoint. Then, LGWR sends DBWR list of modified buffers that have to be written on the disks.

2.
Yes, the scenario where some data are written on the disks, but instances crashed before LGWR flushed log buffers to redo log files is possible. Lets say that happened for an uncommitted transaction X.

During the instance recovery, smon will roll forward all data up to the consistent SCN for all transactions. That latest SCN is the SCN from the redo log files. After the "rolling forward", the old rollback data (with the changes from transaction X) will still be in the database. During the "rolling backwards", oracle looks at the outstanding transactions from rollback segments (one of them is transaction X) and rolls back them. So, your data from transaction X will be lost.

Regards,
Tanya
--
Tanya Injac
Oracle Developer/DBA
Unisys NZ

Umar FArooq <umar.farooq_at_cressoft.com.pk> wrote in article <355ADCE8.A5A3FE20_at_cressoft.com.pk>...
> Can someone please tell me how LGWR and DBWR interact...do they flush
> their respective buffers simultaneously....what happens if DBWR has
> flushed the data and the system crashes before LGWR could flush the redo
> buffer...is such a scenario possible?
>
> Thx.
>
> Umar Farooq Ch.
> umar.farooq_at_cressoft.com.pk
>
>
Received on Thu May 14 1998 - 21:25:56 CDT