Re: buffer busy waits and v$filestat

On 2003.07.29 21:36, Prem Khanna J wrote:
> Gogala,
>
> <quote>
> RMAN is writing the block. That's right, RMAN locks (pins) blocks in
> memory, otherwise it couldn't ensure consistent backup. That is the
> reason
> why RMAN doesn't need "alter tablespace begin backup" command.
> </quote>
>
> I have read that RMAN has the advantage of not generating excessive redo
> which "alter tablespace begin backup" does.
>
> When RMAN locks a data block while backup and suppose if some data is
> to be written to the same data block,what happens now ?

The answer is very simple: write will wait until block is released ("un-pinned"). Locks, latches and pins exist to prevent data corruption from the simultaneous access. Those mechanisms are, essentially, serializing the access to the critical resources. There is a very nice book, an oldie but goodie, that I read once upon a time, called "Modern Operating Systems", written by Andrew Tannenbaum.

>
> 1.where will the data be until RMAN releases the lock ?
> 2.will this be a wait event.if so,how can it be identified ?

Data will wait in user buffers. Yes, it will be a wait event, and same with the DBWR, when the buffer is pinned, it will be a "buffer busy", and when I/O is in progress, it will be "write complete".

>
> Sorry,if i sound silly.
> i am going thro' the RMAN book by Robert Freeman and so curious to know ?!

Bob Freeman's book is an excellent book which helped me understand and write a few RMAN scripts. It is neither intended nor very good at thorough explanation of oracle kernel. For that, there is Steve Adams and his book. I'd also like to abuse this occasion and ask Steve whether he plans a revised V9 (or 10x) edition?

>
> Regards,
> Jp.
>
> 30-07-2003 06:49:23, Mladen Gogala <mgogala_at_adelphia.net> wrote:
> >Buffer busy wait has a different correlation with v$filestat and I/O. Buffer
>
> >busy wait simply means that the buffer you're waiting for is pinned by
> >somebody else.
> >There are 3 classic situations:
> >1) DBWR hasn't finished writing to the disk yet.
> >2) Block is locked by another node (OPS, RAC).
> >3) RMAN is writing the block. That's right, RMAN locks (pins) blocks in
> > memory, otherwise it couldn't ensure consistent backup. That is the
> reason
> > why RMAN doesn't need "alter tablespace begin backup" command.
> >To make the long story short, there is a note on metalink (Note:155971.1)
> with
> >an appropriate title: Resolving Intense and "Random" Buffer Busy Wait
> >Performance Problems. Buffer busy waits are usually a consequence of I/O
> >subsystem not being to provide enough throughput to the database. What can
> you
> >do with v$filestat? You can find where are your hot spots and fix the
> problem.
>
>
>

-- 
Mladen Gogala
Oracle DBA
-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Mladen Gogala
  INET: mgogala_at_adelphia.net

Fat City Network Services    -- 858-538-5051 http://www.fatcity.com
San Diego, California        -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from).  You may
also send the HELP command for other information (like subscribing).