Binay,
from what i understand, these type of block reads
relate to read consistancy .. meaning that if oracle
wants to read block x but it finds that it is dirty ,
it reads from the rollback segments to give you the
point in time snapshot as it existed at the time when
you had first started the query.
e.g. u fire query at 12.00 and if at 12.05, someone
changes a block and commits. Now at 12.10 your query
is still executing and it requests the above block,
oracle will attempt to read it from the rbs , therby
resulting in a consistent read.
a current mode read on the other is a direct read off
of the buffer cache.
correct me if i am wrong here guys?
Thx
Deepak
--- [EMAIL PROTECTED] wrote:
> Hi Deepak,
> Can you elaborate what is consistent gets
>
>
>
> -----
> Eric,
>
> here's a high level from my understanding on this
> issue:
>
> Buffer gets {also called Logical IO's}
> These happen as oracle scans blocks of data in the
> buffercache(in-mem scans). Many people believe that
> since these are memory reads, they are inexpensive.
> I
> have seen the contrary in many cases and have seen
> that these are the ones which take the most cpu
> clycles, therby making your system CPU Bound.
> Resolution of this is to tune your sql by having it
> use better access paths (indexes). Also consider
> de-norming in ordr to avoid too many joins
> I consider this the most important metrics in
> identifying "bad" SQL. i have seen cases where
> frequently executed queries were performing millions
> of LIO's and hosing up the CPU. A simple index / or
> Adding hint can reduce this number by a very high
> factor resulting in great gains.
>
> DiskReads {also called physical IO's)
> This obviously means that there are a lot of disk
> reads required to satisfy your query. Reasons: maybe
> you are using ineffcient access paths/bad sql or u
> just have insuffient (small) memory to support your
> app. High Diskreads is the reason that makes your
> system IO bound. Resolution is again the same as
> described above. In addition, one of the assumtions
> here is that you have spread your datafiles/logs/cf
> optimally. Also consider using the recycle buffer
> pool
> feature to avoid an innocent FTS from flushing
> everything from your cache. Obviously you cannot
> always prevent any of these and some disk read are
> inevitable.
>
> hth
> Deepak:
>
> --- Erik Williams <[EMAIL PROTECTED]> wrote:
> >
> > I am trying to identify the most harmful
> statements
> > in an application. From
> > the Oracle Performance and Tuning Tips and
> > Techniques book, I found two
> > statements. Both are looking at the statements
> > contained in the v$sqlarea.
> > The first looks at statements with a high number
> of
> > buffer gets and the
> > other looks at the statements with a high number
> of
> > disk reads. Some of the
> > statements appear in both lists, but some in only
> > one. If all of the disk
> > reads are moving blocks into the buffer cache,
> what
> > is the difference
> > between the two measures? Can anyone explain the
> > difference between the two
> > measures?
> >
> > Thanks.
> > Erik
> >
> > --
> > Please see the official ORACLE-L FAQ:
> > http://www.orafaq.com
> > --
> > Author: Erik Williams
> > INET: [EMAIL PROTECTED]
> >
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--
Author: Deepak Thapliyal
INET: [EMAIL PROTECTED]
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