Storage Administration - Oracle Exadata Recipes: A Problem-Solution Approach - page 286

Now we'll run the same tests against our MMT_NOSS table, which resides on an ASM disk
group with cell.smart_scan_capable set to FALSE .

10.

APPS @ visx1> select count(1) from (
select /*+ full (x) */ *
from mmt_noss x)
/

COUNT(1)
----------
8783431
Elapsed time: 16.61
APPS @ visx1> @lst09-18-examystats.sql
Statistic Statistic Value
------------------------------ -----------------
MB Requested 3277.79
MB Eligible for Offload 0.00
Smart Scan Efficiency 0.00
Interconnect MB 3273.68
... Output omitted for brevity

As you can see, zero bytes were eligible for Smart Scan, the offload efficiency was zero, and the query returned in
16.61 seconds.

How It Works
Setting the cell.smart_scan_capable ASM disk group attribute to TRUE will enable Smart Scan for qualifying SQL
queries that access segments contained in the disk group, and setting the attribute to FALSE will disable Smart Scan.
Exadata's storage server software examines this ASM disk group attribute as it ingests the iDB-encapsulated I/O
request metadata from the SQL statements on the compute nodes and uses this attribute to determine whether to
perform offload operations or act as a traditional block I/O server.
In real-world scenarios, we rarely see a non-academic business justification for setting cell.smart_scan_capable
to FALSE . Exadata's most powerful performance feature is Smart Scan; disabling Smart Scan removes Exadata's biggest
performance tool from the toolbox. This being said, using the ASM disk group cell.smart_scan_capable parameter
can arguably be the most simple way to simply “turn off ” Smart Scan, which in some cases may be interesting to do if
you are conducting a “what if ” analysis.

Not ■ Use caution when disabling Smart Scan at the aSm disk group level and make sure to enable it unless you
have a strong business justification to leave cell.smart_scan_capable set to FALSE . in our experience, aSm disk group
attribute settings are one layer too deep in the technology stack for most traditional oracle database administrators to
consider when troubleshooting performance problems, and it can be maddening to try to figure out why Smart Scan isn't
working on your exadata environment if you have disabled it for an aSm disk group.