Re: buffer busy waits and v$filestat

Thanks, Mladen, for that helpful quick reply.

The reason I ask is, one of our clients claims that they see a intermittent database performance degrade (queries get timed out), during times when the RMAN backup runs. Using an OS monitoring tool, they see a spike in I/O.

The RMAN backups have 4 channels writing to disk, and as such, I/O is to be expected. I generated statspack reports during the times the backup runs, and the tablespace I/O summary does show high avg reads/ms. Also, I see higher than normal "total waits" in the tablespace I/O summary. This, I guess, should be the "buffer busy wait" events, though I dont see it amongst the top 15 wait events. Also, the "data block" waits in v$waitstat spikes during this backup. So, I was wondering if I these wait events are the real cause of the I/O spike?

Thanks

Raj

                                                                                                                   
                    Mladen Gogala                                                                                  
                    <mgogala_at_adelp       To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>       
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                    Sent by:             Subject:     Re: buffer busy waits and v$filestat                         
                    ml-errors_at_fatc                                                                                 
                    ity.com                                                                                        
                                                                                                                   
                                                                                                                   
                    07/29/2003                                                                                     
                    05:49 PM                                                                                       
                    Please respond                                                                                 
                    to ORACLE-L                                                                                    
                                                                                                                   
                                                                                                                   

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.

On 2003.07.29 17:24, Rajesh.Rao_at_jpmchase.com wrote:
> Folks,
>
> Say a session issues a read request, and finds another session already
> reading the block into the buffer cache. If this session waits N ms on a
> "buffer busy waits" event, does this N ms of wait get added to the read
> times in v$filestat? Or is the readtim in v$filestat purely physical
I/O?
>
> Thanks
> Raj
>

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