Smart Flash Cache and Smart Flash Logging - Oracle Exadata Recipes: A Problem-Solution Approach - page 537

You may be asking yourself the following question: “Should I pin objects in flash or allow Smart Flash Cache
to make its own judgments about whether a segment should be cached or not?” Like many performance-related
configuration decisions, it truly depends on your workload and performance SLAs. There are cases when specific
segments have very stringent performance requirements that cell offload alone may not be able to provide, hence
motivating you to KEEP large segments. More commonly, however, are situations in very busy environments in which
OLTP-type I/O access requests need to be satisfied from flash, and the data access patterns are such that the data is very
uncommonly accessed, such as a month-end period close process. In cases such as these, the blocks required to satisfy
a program may be unlikely to reside in Oracle's database buffer cache, and it may be beneficial to KEEP these in flash.

18-7. Quantifying Benefits of Smart Flash Logging
Problem
You are experiencing performance issues related to heavy redo log write activity and wish to quantify the benefit of
Exadata's Smart Flash Logging behavior, introduced in storage cell version 11.2.2.4.2.

Solution
In this recipe, you will learn how to query Exadata's Smart Flash Logging ( FLASHLOG ) metrics in order to measure the
performance impact and redo write I/O savings provided by Smart Flash Logging. To demonstrate the behavior of
Smart Flash Logging, we will execute a simple anonymous PL/SQL block that issues 20 million single-row inserts and
performs 20 million single commits:

SQL> declare
myrec d14.myobj_uncomp%rowtype;
cursor mycursor is select * from d14.myobj_uncomp;
begin
for myrec in mycursor loop
insert into d14.myobj_u2
values myrec;
commit;
end loop;
end;
/

■ if you have an application that is actually designed in this manner, we advise sitting down with the developers
and discussing alternative strategies.

Note

To measure the impact of Smart Flash Logging, query the FL_DISK_FIRST , FL_FLASH_FIRST , FL_ACTUAL_OUTLIERS ,
and FL_PREVENTED_OUTLIERS FLASHLOG metrics. The script in Listing 18-8 summarizes this information.

#!/bin/sh
# Name: lst13-10-flashlog-eff.sh
# Purpose: Show Smart Flash Logging effiency
# Usage: ./lst13-10-flashlog-eff.sh
# Adjust the following to suit your needs
CELLGROUP=/home/oracle/cell_group