which two reasons would the; “physical read total bytes” statistic be greater than the “cell physical IO bytes eligible for predicate offload” statistic?

You are evaluating the performance of a SQL statement that accesses a very large table, and
have run the following query producing the output shown:

For which two reasons would the; “physical read total bytes” statistic be greater than the “cell
physical IO bytes eligible for predicate offload” statistic?

You are evaluating the performance of a SQL statement that accesses a very large table, and
have run the following query producing the output shown:

For which two reasons would the; “physical read total bytes” statistic be greater than the “cell
physical IO bytes eligible for predicate offload” statistic?

A.
There is an index on the column used in the where clause, causing “cell multiblock physical
reads” to be requested by the database instance, resulting in additional I/O for blocks in the cells.

B.
The table is an IOT and has an overflow segment, causing “cell multiblock physical reads” to be
requested by the database instance, resulting in additional I/O for block in the cells.

C.
There is an uncommitted transaction that has modified some of the table blocks, causing some
“cell single block physical reads” to be requested by the database instance, resulting in additional
I/O for block in the cells.

D.
The table is an indexed clustered table, causing “cell single block physical reads” to be
requested by the database instance, resulting in additional I/O forblocks in the cells.

E.
There are migrated rows in the table, causing some “cell single block physical reads” to be
requested by the database instance, resulting in additional I/O for blocks in the cells.

Explanation:
Note:
* physical read total bytes: the size of the segment to read is known by the database, and must be
read entirely from the database’s perspective.
*cell physical IO bytes eligible for predicate offload: this statistic shows the amount of data which
the cell server is able to process on behalf of the database, instead of the database processing
and the cell server just delivering blocks.
*Cell physical IO bytes eligible for predicate offload — This number should be high
The higher the number more MB/GB is filtered out at the cell level itself rather sending it to the
buffer cache to filter the rows.
*In this case, all bytes are processed on the cellserver (cell physical IO bytes eligible for predicate
offload=physical read total bytes)
*Cell Offloading:-The storage cells are intelligent enough to process some workload inside them, saving the
database nodes from that work. This process is referred to as cell offloading.



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Miles

Miles

I believe the answer is C & E.

Rumi

Rumi

Smart Scan does not work on IOT and Cluster Tables. What about C & D?

Rumi

Rumi

Sorry, typo! I meant B & D as correct answers.

UP

UP

C is definitely correct answer.

Sanjay Mevada

Sanjay Mevada

C is correct answer. Not sure about 2nd one.

nohup

nohup

‘cell multiblock physical read’:

This Wait Event occurs most likely because of a conventional Full Table Scan resp. a conventional Index Fast Full Scan (both not offloaded). Typically because the Table resp. the Index was small. It would be surprising to see this as a major Wait Event on a Data Warehouse Database running on Eaxadata, but it will show up regularly on OLTP running on Exadata

‘cell single block physical read’:

The most likely reason for this is a conventional (not Offloaded) Index Access. Again, we expect to see this Wait Event high on OLTP but low on Data Warehouse Databases, running on Exadata

nohup

nohup

Some smartscan disablers

. IOTs
. Clustered Tables
. Rowdependencies Enabled
. LOBs
. No Direct Path Reads

So I think it should be B D

nohup

nohup

sorry, I meant it should NOT be B D

nohup

nohup

I’ll go with C E

L. Zhu

L. Zhu

A is wrong. Index scan won’t use Smart Scan. but offload shows smart scan in use.
B is wrong. IOT won’t use Smart Scan. but offload shows smart scan in use.
C is right. read consistency will interrupt Smart Scan and causing more cell single block physical read.
D is wrong. Index clustered table won’t use Smart Scan. but offload shows smart scan in use.
E is right. migrated row will interrupt Smart Scan and causing more cell single block physical read.

so C.E. are correct

Amit

Amit

C & E

Smart scan won’t work for IOT and clustered tables

Shrikant.Homkar

Shrikant.Homkar

“physical read total bytes” statistic be greater than the “cell physical IO bytes eligible for predicate offload” statistic. To make this happen Smart Scan should not be used… if yes.. then correct answer would be B and D

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Carlos

Carlos

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