What is Exadata Write-Back Flash Cache?
Exadata Write-Back Flash Cache provides the ability to cache not only read I/Os but write I/O to the Exadata’s PCI flash on the storage cells. Exadata storage software 220.127.116.11.1 or higher and Grid Infrastructure and Database software 18.104.22.168.9 or higher is required to use Exadata Write-Back Flash Cache, which is persistent across storage cell restarts.
The default since April 2017 for the Oracle Exadata Deployment Assistant (OEDA) is Write-Back Flash Cache when DATA diskgroup is HIGH redundancy and Grid Infrastructure and Database software are:
- 22.214.171.124.1 or higher
- 126.96.36.199 or higher
- 188.8.131.52 or higher
PLEASE NOTE: This option is only applicable to High Capacity as Extreme Flash doesn’t have Hard Disks and therefore Write-Back Flash Cache is explicitly enabled and can’t be disabled.
What are the Performance Benefit of Exadata Write-Back Flash Cache?
Write-Back Flash Cache can significantly improve write intensive operations because writing to Flash Cache is significantly faster than writing to Hard Disks. Depending on the workload, write performance (IOPS) can be improved by 10x on older generations of Exadata Machines V2 and X2 and 20x on newer generations X3 onwards (correct at time of writing).
If you are experiencing high write I/O times on storage cells from AWR Reports or Storage Cell metrics, then you should consider enabling Write-Back Flash Cache to alleviate write operations on Hard Disks and move to Flash Cache.
See the following My Oracle Support (MOS) Note for more info:
Exadata Write-Back Flash Cache – FAQ (Doc ID 1500257.1)
How to check if Exadata Write-Back Flash Cache is Enabled?
To check if Exadata Write-Back Flash Cache is enabled, run “list cell attributes flashcachemode” on the storage cell using CellCLI as shown below:
[root@v1ex2celadm01 ~]# cellcli CellCLI: Release 184.108.40.206.5 - Production on Wed Jan 17 10:09:51 GMT 2018 Copyright (c) 2007, 2016, Oracle. All rights reserved. CellCLI> list cell attributes flashcachemode WriteThrough CellCLI> exit quitting [root@v1ex2celadm01 ~]#
If “WriteThrough” then Write-Back Flash Cache is disabled (writes go straight to hard disk and then can be placed in flash for caching reads if required), otherwise if “WriteBack” then Write-Back Flash Cache is enabled as the name suggests (writes go straight to flash and then can be moved to hard disk if aged or not required for read caching).
You can also run “list cell detail” using CellCLI as shown below:
[root@v1ex2celadm01 ~]# cellcli CellCLI: Release 220.127.116.11.5 - Production on Wed Jan 17 10:10:22 GMT 2018 Copyright (c) 2007, 2016, Oracle. All rights reserved. CellCLI> list cell detail name: v1ex2celadm01 accessLevelPerm: remoteLoginEnabled bbuStatus: normal cellVersion: OSS_18.104.22.168.5_LINUX.X64_170418 cpuCount: 16/32 diagHistoryDays: 7 eighthRack: TRUE fanCount: 8/8 fanStatus: normal flashCacheMode: WriteThrough id: xxxxxxxxxx interconnectCount: 2 interconnect1: ib0 interconnect2: ib1 iormBoost: 6.4 ipaddress1: 10.1.11.14/22 ipaddress2: 10.1.11.15/22 kernelVersion: 2.6.39-400.294.4.el6uek.x86_64 locatorLEDStatus: off makeModel: Oracle Corporation ORACLE SERVER X5-2L High Capacity memoryGB: 95 metricHistoryDays: 7 notificationMethod: snmp notificationPolicy: critical,warning,clear offloadGroupEvents: powerCount: 2/2 powerStatus: normal releaseImageStatus: success releaseVersion: 22.214.171.124.5.170418 rpmVersion: cell-126.96.36.199.5_LINUX.X64_170418-1.x86_64 releaseTrackingBug: 25509078 rollbackVersion: 188.8.131.52.4.170111 securityCert: PrivateKey OK Certificate: Subject CN=v1ex2celadm01.v1.com,OU=Oracle Exadata,O=Oracle Corporation,L=Redwood City,ST=California,C=US Issuer CN=v1ex2celadm01.v1.com,OU=Oracle Exadata,O=Oracle Corporation,L=Redwood City,ST=California,C=US snmpSubscriber: host=v1ex2dbadm02.v1.com,port=1830,community=public host=v1ex2dbadm01.v1.com,port=1830,community=public host=v1ex2dbadm01.v1.com,port=3872,community=public host=v1ex2dbadm02.v1.com,port=3872,community=public status: online temperatureReading: 24.0 temperatureStatus: normal upTime: 105 days, 7:35 usbStatus: normal cellsrvStatus: running msStatus: running rsStatus: running CellCLI> exit quitting [root@v1ex2celadm01 ~]#
However, the simpler way to check is via dcli, especially when you have lots of storage cells as shown below:
[root@v1ex2dbadm01 ~]# dcli -g /opt/oracle.SupportTools/onecommand/cell_group -l root cellcli -e "list cell attributes flashcachemode" v1ex2celadm01: WriteThrough v1ex2celadm02: WriteThrough v1ex2celadm03: WriteThrough
How to Enable Exadata Write-Back Flash Cache
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Zed DBA (Zahid Anwar)