Oracle Exadata Smart Flash Logging

What is Exadata Smart Flash Logging?

In an OLTP environment, it is crucial to have fast response times to redo log writes i.e. low latency.  When multiplexing redo logs for high availability i.e. to protect against hardware failure, redo log writes are only acknowledged when redo is written to all redo log members i.e when the slowest disk completes the write.  By this nature, whenever a disk slows down even if for a moment it can have impact on redo log performance and throughput.

Flash alone can’t resolve this issue as flash can also momentarily slow down due to issues in erase cycles or wear leveling and remember the acknowledgement is only given when the redo is written to all redo log members.

Exadata Smart Flash Logging, is the feature that writes to both hard disk and flash with the acknowledgement given as soon as either completes the write, thus improving response time and throughput.  So if a write is slow to hard disk the flash will give a quicker acknowledgement but when flash is experiencing a slow down due to erase cycles or wear leveling then the hard disk will acknowledge, smoothing out response times.

The Exadata Smart Flash Cache isn’t permanent but a temporary store to provide fast response times by storing redo until it’s safely written to disk.

No changes are required to redo log configuration and is transparent to database and recovery.

How to enable Smart Flash Logging?

It’s enabled out the box or for older systems it’s enabled when applying cell patch version 11.2.2.4 and also requires Database 11.2.0.2 Bundle Patch 11 or higher.

How to disable Smart Flash Logging?

This shouldn’t be done unless instructed to do so by Oracle Support or Development.

How much flash is used by Smart Flash Logging?

By default just 512Mb is used per cell, which should be sufficient for most situations.   It’s a small investment for huge performance benefit.  Statistics record the number of successful write and unsuccessful writes due to the temporary space filled.  In which case the size may need to be increased.  Also I/O Resource Manager (IORM) can be used to disable Smart Flash Logging for none critical databases.

Do standby redo logs use Smart Flash Logging?

Yes, standby redo logs benefit from Smart Flash Logging just as redo logs as long as cell patch 11.2.2.4 or higher is applied and Database 11.2.0.2 Bundle Patch 11 or higher is applied.

How to check that Smart Flash Logging is configured?

Using CellCLI run “LIST FLASHLOG DETAIL” and if output is returned as shown below with the details, then this means that Smart Flash Logging is configured:

[root@v1ex1dbadm01 ~]# dcli -g /opt/oracle.SupportTools/onecommand/cell_group -l root cellcli -e "list flashlog detail"
 v1ex1celadm01: name: v1ex1celadm01_FLASHLOG
 v1ex1celadm01: cellDisk: FD_00_v1ex1celadm01,FD_01_v1ex1celadm01
 v1ex1celadm01: creationTime: 2015-06-28T17:52:43+01:00
 v1ex1celadm01: degradedCelldisks:
 v1ex1celadm01: effectiveSize: 512M
 v1ex1celadm01: efficiency: 100.0
 v1ex1celadm01: id: 366421ec-bf77-499e-870f-f0cf5390343e
 v1ex1celadm01: size: 512M
 v1ex1celadm01: status: normal
 v1ex1celadm02: name: v1ex1celadm02_FLASHLOG
 v1ex1celadm02: cellDisk: FD_01_v1ex1celadm02,FD_00_v1ex1celadm02
 v1ex1celadm02: creationTime: 2015-06-28T17:52:44+01:00
 v1ex1celadm02: degradedCelldisks:
 v1ex1celadm02: effectiveSize: 512M
 v1ex1celadm02: efficiency: 100.0
 v1ex1celadm02: id: 9f670843-c9cc-4156-a32e-8d23fa79cdb8
 v1ex1celadm02: size: 512M
 v1ex1celadm02: status: normal
 v1ex1celadm03: name: v1ex1celadm03_FLASHLOG
 v1ex1celadm03: cellDisk: FD_01_v1ex1celadm03,FD_00_v1ex1celadm03
 v1ex1celadm03: creationTime: 2015-06-28T17:52:33+01:00
 v1ex1celadm03: degradedCelldisks:
 v1ex1celadm03: effectiveSize: 512M
 v1ex1celadm03: efficiency: 100.0
 v1ex1celadm03: id: 749bada6-8ae2-4c51-8410-97622f9a9532
 v1ex1celadm03: size: 512M
 v1ex1celadm03: status: normal
[root@v1ex1dbadm01 ~]#

For more info:
Exadata Smart Flash Logging FAQ (Doc ID 1372894.1)
Oracle Exadata Whitepaper:  Exadata Smart Flash Cache Features and the Oracle Exadata Database Machine

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Thanks

Zed DBA (Zahid Anwar)

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SNMP unresponsive on Storage Cell after Exadata Patching

After a round of Exadata patching, the Storage Cells become unresponsive to SNMP for monitoring tools like OpsViews:

check_snmp_sysinfo CRITICAL - Agent not responding, tried SNMP v1 and v2c

This is because the Storage Cells get re-imaged as part of the Exadata patching, confirmed by imagehistory:

[root@v1ex1celadm01 ~]# imagehistory
Version : 12.1.2.1.1.150316.2
Image activation date : 2015-05-01 16:10:10 -0700
Imaging mode : fresh
Imaging status : success

Version : 12.1.2.1.3.151021
Image activation date : 2015-12-16 05:25:21 +0000
Imaging mode : out of partition upgrade
Imaging status : success

Version : 12.1.2.2.1.160330
Image activation date : 2016-05-12 14:02:43 +0100
Imaging mode : out of partition upgrade
Imaging status : success

Version : 12.1.2.3.2.160721
Image activation date : 2016-10-05 01:45:42 +0100
Imaging mode : out of partition upgrade
Imaging status : success

Version : 12.1.2.3.3.161208
Image activation date : 2017-01-18 02:53:10 +0000
Imaging mode : out of partition upgrade
Imaging status : success

Version : 12.1.2.3.4.170111
Image activation date : 2017-05-23 10:58:24 +0100
Imaging mode : out of partition upgrade
Imaging status : success

[root@v1oex1celadm01 ~]#

Which knocks out the configuration in iptables and snmpd.conf.

To resolve add back in the lines for your primary and secondary monitoring servers, for example:

rocommunity V12V1 192.168.0.31
rocommunity V12V1 192.168.0.32

So looks like this:

[root@v1ex1celadm01 ~]# more /etc/snmp/snmpd.conf
trapcommunity public
trapsink 127.0.0.1 public
rocommunity public 127.0.0.1
rocommunity V12V1 192.168.0.31
rocommunity V12V1 192.168.0.32
rwcommunity public 127.0.0.1

access RWGroup "" any noauth exact all all all
com2sec snmpclient 127.0.0.1 public
group RWGroup v1 snmpclient

pass .1.3.6.1.4.1.4413.4.1 /usr/bin/ucd5820stat
pass .1.3.6.1.4.1.3582 /usr/sbin/lsi_mrdsnmpmain

syscontact Root <root@localhost> (configure /etc/snmp/snmp.local.conf)
syslocation Unknown (edit /etc/snmp/snmpd.conf)

view all included .1 80

[root@v1ex1celadm01 ~]#

Then reload snmp using the user root:

[root@v1ex2celadm01 ~]# service snmpd reload
Reloading snmpd: [ OK ]
[root@v1ex2celadm01 ~]#

Next add the monitoring servers primary and secondary to iptables as follows:

[root@v1ex1celadm01 ~]# iptables -I INPUT -s 192.168.0.31 -p udp --dport 161 -j ACCEPT
[root@v1ex1celadm01 ~]# iptables -I INPUT -s 192.168.0.32 -p udp --dport 161 -j ACCEPT

And make permanent by saving to firewall rules:

[root@v1ex1dbadm01 ~]# /etc/init.d/iptables save
iptables: Saving firewall rules to /etc/sysconfig/iptables:[ OK ]
[root@v1ex1dbadm01 ~]#

Now your monitoring should return as SNMP connectivity is restored 🙂 :

SNMP OK - "Linux v1ex1celadm01.v1.com 2.6.39-400.294.1.el6uek.x86_64 #1 SMP Wed Jan 11 08:46:38 PST 2017 x86_64"

 

If you found this blog post useful, please like as well as follow me through my various Social Media avenues available on the sidebar and/or subscribe to this oracle blog via WordPress/e-mail.

Thanks

Zed DBA (Zahid Anwar)