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Re: Database on the file system

From: Yugos <yugos_at_d1.dion.ne.jp>
Date: Wed, 30 Jun 1999 02:00:45 GMT
Message-ID: <7lbtoa$1jb$1@nnrp1.deja.com>


Hi, Nuno,

Thank you for your comment.

In article <7kvvip$haf$1_at_m2.c2.telstra-mm.net.au>,   "Nuno Souto" <nsouto_at_nsw.bigpond.net.au> wrote:
> Guys,guys, please! fsck looses files only when they have grown
> and the growth hasn't reached the disk's master tables before the
crash!
> Otherwise, fsck does not "loose" files. And what does ORACLE use
> for storing its data? Fixed length files in most cases anyway.
Certainly
> in the case of the redo logs. So if system crashes, no redo log file
lost
> anywhere! Simple.

Yes, as you said, fixed length files will be protected in fsck phase but how about archive log files ?@redolog files are switching periodically and flushed to archive log directory. If I met system crash during archive redolog files, some of redolog files may be lost in next fsck phase.
In fact, I see some junk files that were no doubt redolog files in archive log directory, in lost+found directory after fsck.

>
> Also, note that most modern file systems accept some form of "write-
through:,
> ie, the fs lets the software write the file and returns only when the
write is
> finished, instead of the old "I'll write it later but consider it
done"
> technology.
> ORACLE uses this kind of stuff for the redo log files. Veritas works
slightly
> different than this, but the end result is the same.
>
> Raw just bypasses the fs cache and all the other fs rigmarole that
goes with it
> and lets ORACLE "write to the bone", so to speak. In theory, can be
faster.
> But you need to have the ORACLE cache ready to replace the speed
> lost by not using the fs cache. And a few other less better known
> ORACLE caches as well.
>
> In simple terms, the above describes MOL what happens. There is
actually
> a lot of meat to go through in a subject like this, but this pretty
much sums
> it up.
>
> --
> Cheers
> Nuno Souto
> nsouto_at_nsw.bigpond.net.au.nospam
> Is there a nospam domain?
> http://www.users.bigpond.net.au/the_Den
> Bass Chorng <bass_at_octel.com> wrote in message
> news:7ku4ne$kd8$1_at_news.eng.octel.com...
> > : 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...
> >
> > 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.
>
>

Thanks,
Yugos
yugos_at_d1.dion.ne.jp

Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't. Received on Tue Jun 29 1999 - 21:00:45 CDT

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