Re: Database on the file system

Yugos (yugos_at_d1.dion.ne.jp) wrote:
: Hi, Oracle DBA

: I have a simple question.

: If I choose to put redolog files and database files on the file system,
: instead of raw devices and when I meet the system crash trouble,
: I wonder why Oracle can completely make good to recover all database
: files by redolog files.

: I mean, as you may know, data-writing to all files on the file system
: is generally kept once on kernel cash area so-called the file system
: buffer and contents in the buffer are periodically flushed to the
: physical disks by a syncer daemon.
: Considering that system crash happen perior to buffer flush, I guess
: some redolog files may be lost in next fsck phase. (Especially in
: high-transaction environment, redolog file switching is also high.)
: In case of loosing redolog files application no longer recover its
: database, I think. It means it may lost some (many?) transactions.

: I wonder why really safe way to put oracle files on the file systems,
: instead of raw devices...

: Thanks,
: Yugo Shimada
: yugos_at_d1.dion.ne.jp

Interesting point of view. I dont have an answer, but like to add another question. Isn't this problem still true even when you use raw ? Because when OS 'thinks' a write is commited, it actually has to go thru controller which has buffer and disk which also has buffer, so you can still lose transaction if a power outage happens at the right time. Bear in mind, it's a long way before it is finally physically writen to disk, unless you use subsystem like EMC which has battery powered cache. Loss of commited data is always possible, although chance is slim.

Among all the debates between raw and cooked file system I have never heard any discussion on this respect. But all I know is we use filesystem for over 45 instances for years and have gone thru crashes and sudden reboots ( while transaction is happening ), the loss of transaction in redo has never happened.

-Bass Chorng Received on Thu Jun 24 1999 - 15:33:50 CDT