Re: v$sess_io

In article <vGrWNyUf+tykOOoqq39d8drP+oJX_at_4ax.com>,   Christopher Spence <cspence_at_delphi-tech.com> wrote:
> If Block gets are the number of blocks read to fill session requests,
> why is it lower than consistant? I would think it would be equal to
> consistant or bigger.
>
> How do you read this view for performance?
> I.e. if x number is higher than x this is a problem.
>
> Is this covered in performance tuning course? I take that one in a
> couple of weeks.
>
> Oracle ID Block Gets Consistent Gets
> Physical Reads Block Changes Consistent Changes
> ------------------------------ ---------------- ----------------
> ---------------- ---------------- ------------------
> ACEQA 909 79,723
> 8,318 9 0
>
> For example here, Block gets are 909 meaning oracle needed to retrieve
> 909 blocks (from buffer and directly from disk for fts) to satisfy all
> the queries. 79,723 blocks where looked at while pending changes?
> (Thus high level of modifications being made?)
>
> How do you put it all together? the relationship of block gets and
> consistant gets confuses me how block gets is lower.
>
> On Wed, 8 Sep 1999 14:46:34 +0100, "Neilc"
> <neilc-olops_at_btinternet.com> wrote:
>
> >v$sess_io lists I/O statistics for each user session.
> >
> >block gets are the number of database blocks read to satisfy a
particular
> >session.
> >consistent_gets are the number of reads from rollback_segments
> >physical_reads are reads of data blocks into memory from disk.
> >
> >Hope this helps
> >
> >Neil C
> >
> >Christopher Spence wrote in message ...
> >> Could someone please explain accurately what exactly 'BLOCK
> >>GETS' and 'CONSISTANT GETS' refers to under this view? I believe i
> >>know wha they mean but if someone that is 100% sure can explain this
> >>that would be wonderful. Also 'Physical Reads' would be great
too. I
> >>have heard different answers to this.
> >>
> >

I need to disagree with part of Chris's reply. I believe that consistent gets in v$sess_IO represents the number of consistent (row) reads that were requested. The consistent_changes column is the number of rollback entries that had to be applied to get consistent reads.

You may want to see appendix C of the ver 8 Reference manual for a list of Oracle statistics for v$sysstat. Hopefully, Oracle is consistent across v$ tables with its definitions (there were some known inconsistencies in ver 7.3 and below which probably still exist.)

--
Mark D. Powell -- The only advice that counts is the advice that  you follow so follow your own advice --

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Share what you know. Learn what you don't. Received on Thu Sep 09 1999 - 08:48:36 CDT