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v$sess_io

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ericfjchen
Regular Advisor

v$sess_io

The v$sess_io view has 2 columns as below,

CONSISTENT_GETS
CONSISTENT_CHANGES

May you explain them?

Thanks
6 REPLIES
harry d brown jr
Honored Contributor

Re: v$sess_io

Try this:

http://www.oracle-training.cc/phys_56.htm

live free or die
harry d brown jr
Live Free or Die
harry d brown jr
Honored Contributor

Re: v$sess_io

Or

http://www.lc.leidenuniv.nl/awcourse/oracle/server.920/a96536/ch3169.htm

found with a simple google

live free or die
harry d brown jr
Live Free or Die
Jean-Luc Oudart
Honored Contributor

Re: v$sess_io

Hi Eric

check this Metalink document
557629.994 consistent_changes real high

I would recommend you use statspack reports for a snapshot view as the view reflect total since the start of the instance.

Regards
Jean-Luc
fiat lux
TwoProc
Honored Contributor

Re: v$sess_io

consistent gets may be thought of data base gets which have sucessfully come from the db_cache (not from disk), similarly consistent changes were put to cache, and didn't have to involve disk i/o.
We are the people our parents warned us about --Jimmy Buffett
Indira Aramandla
Honored Contributor

Re: v$sess_io

Hi Eric,

$SESS_IO view lists I/O statistics for each user session. It will have 1 row for each session connected to the database, and it tracks logical and physical I/O.

Block gets - logical i/o, current mode, usually DML activity.
Consistent gets - logical i/o, consistent mode, usually SELECT statements
Physical reads - physical i/o, if a block get or consistent get resulted in a cache miss, it caused a physical i/o.
Block changes - logical i/o, how many changes were applied to blocks due to DML. (Changes to current mode blocks)
Consistent changes - logical i/o, how many changes were applied to blocks for read consistency purposes. (Consistent mode changes)

CONSISTENT_CHANGES - Consistent changes for this session
CONSISTENT_GETS - Consistent gets for this session

CONSISTENT_GETS shows the number of blocks requested in consistent mode, for consistent read operations. Consistent gets is the number of blocks requested in consistent mode. Blocks are requested in consistent mode for consistent read operations. When a block is requested in consistent mode, if any changes have been committed or made to that block since the requesting statement (or transaction) began, then they must be rolled back for the purposes of the read, to give a consistent view of the data at that time.

When a block is requested in consistent mode, any changes will be rolled back so the SELECT query can get a read-consistent view of the data as of the time it started.

IA
Never give up, Keep Trying
Yogeeraj_1
Honored Contributor
Solution

Re: v$sess_io

hi,

a simpler explanation for "consistent gets" is:

The number of times a consistent read was requested for a block in the buffer cache. Consistent reads may require read asides to the undo (rollback) information and they will be counted here as well - the reads to the undo.

another important term is "db block gets" which is:
The total number of blocks read from the buffer cache in "CURRENT" mode. Blocks may be retrieved/used by Oracle in one of two ways - "CURRENT" or "CONSISTENT". A current mode get is a retrieve of a block as it exists right now. You will see these most frequently during modification statements since they must update only the latest copy of the block.

Both "db block gets" and "consistent gets" are both logical IO -- they read from the buffer cache.

hope this helps too!
regards
yogeeraj
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