Exadata Flash Cache enabled for Write Back

During Tests for an Migration of a major customer application we saw in our AWR reports that most of the jobs are very write intensive. This was the point where we would like to test what happens when we change the Flash Cache Mode from Write Through to Write Back.

What are the main benefits of Write Back mode:

  • it improves the write intensive operations while writing to flash cache is faster than writing to normal Hard disks
  • on Exadata X3  and newer machines write performance can be improved up to 20X IOPS
  • The Write Back Flash Cache accelerates reads and writes for all workloads

First of all I take a look in Metalink and found a Doc ID 1500257.1 with more details.

What are the requirements?

since April 2017 it is default if the following conditions are full filled:

  • Grid and RDBMS Home
    • 11.2.0.4.1 or higher
    • 12.1.0.2 or higher
    • 12.2.0.2 or higher

and

  • DATA diskgroup has HIGH redundancy

When should I use Write Back?

  • It makes sense if your application is write intensive
  • You find significant waits for „free buffer waits“
  • High IO times to check for write bottlenecks in AWR reports

What are the steps to enable Write Back?

We have the possibility to do it „offline“ so we stop the whole Grid & Rdbms Stack, but you can change it also in a Rolling manner.

  • Actual Flash Cache Mode

dcli -g cell_group -l root "cellcli -e list cell attributes flashcachemode"
cel04: WriteThrough
cel05: WriteThrough
cel06: WriteThrough
cel07: WriteThrough

  • Stop the whole Cluster

crsctl stop cluster -all -f

  • Check State of Flash Cache

name: cel04_FLASHCACHE
status: normal

name: cel05_FLASHCACHE
status: normal

name: cel06_FLASHCACHE
status: normal

name: cel07_FLASHCACHE
status: normal

  • Steps for the Change

These steps has to be done on every Cell Server here as an example

Drop the flash cache on that cell

CellCLI> drop flashcache;
Flash cache cel04_FLASHCACHE successfully dropped.

Shut down Cell service

CellCLI> alter cell shutdown services cellsrv;
Stopping CELLSRV services... The SHUTDOWN of CELLSRV services was successful.

Change Cell Flash Cache mode to Write Back

CellCLI> alter cell flashCacheMode=writeback;
Cell cel04 successfully altered 

Restart the Cell Service

CellCLI> alter cell startup services cellsrv;
Starting CELLSRV services...
The STARTUP of CELLSRV services was successful.

Recreate the Flash Cache

CellCLI> create flashcache all;
Flash cache cel04_FLASHCACHE successfully created

Finally check the State on all Cell Server dcli -g cell_group -l root "cellcli -e list cell attributes flashcachemode"
cel04: WriteBack
cel05: WriteBack
cel06: WriteBack
cel07: WriteBack

So the first step was done and now the tests can go on.

In a few weeks I will give a feedback what are the real improvements so stay tuned.

 

 

 

 

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