I will be presenting in HOTSOS symposium 2014 discussing correct methods to diagnose RAC performance issues. Very surprisingly, even very senior performance engineers make mistakes in their analysis while reviewing RAC issues. Come to my presentation and learn the golden rules of RAC performance diagnostics.
Archive for the ‘RAC’ Category
Posted by Riyaj Shamsudeen on February 25, 2014
Posted by Riyaj Shamsudeen on November 12, 2013
It is easier to create one or two AWR reports quickly using OEM. But, what if you have to create AWR reports for many snapshots? For example, your Oracle support analyst wants you to supply 10 1-hour AWR reports from 10AM to 8PM in a 8 node cluster? That’s about 80 AWR reports to create! Okay, okay, I may(!) be overselling it, but you get the point. It is useful to have a script to create AWR report for all instances for a given range of snapshot IDs. Following scripts are handy:
|1. To create one AWR report per instance, for the last snap duration :||awrrpt_all_gen.sql|
|2. Same as (1) but in html format :||awrrpt_all_genhtml.sql|
|3. To create one AWR report per instance, for a range of snap IDs :||awrrpt_all_range_gen.sql|
|4. To create one AWR report, per instance, per snap ID :||awrrpt_all_multi_gen.sql|
Zip file: awrrpt_scripts
These scripts do not modify anything in the database, just retrieves the data using dbms_workload_repository package. Test the scripts to understand further. Of course, you need access to dbms_workload_repository and access to gv$instance.
Posted by Riyaj Shamsudeen on September 18, 2013
I will be hacking RAC internals with few LINUX tools in Oaktable world presentation series, in SFO. Details are available at Oaktable World 2013
Hope to see you there!
Posted by Riyaj Shamsudeen on September 9, 2013
I blogged about DFS lock handle contention in an earlier blog entry. SV resources in Global Resource Directory (GRD) is used to maintain the cached sequence values. I will further probe the internal mechanics involved in the cached sequences. I will also discuss minor changes in the resource names to support pluggable databases (version 12c).
Let’s create an ordered sequence in rs schema and then query values from the sequence few times.
create sequence rs.test_seq order cache 100; select rs.test_seq.nextval from dual; -- repeated a few times. ... / 21
Sequence values are permanently stored in the seq$ dictionary table. Cached sequence values are maintained in SV resources in GRD and SV resource names follows the naming convention to include object_id of the sequence. I will generate a string using a small helper script and we will use that resource name to search in the GRD.
SELECT DISTINCT '[0x' ||trim(TO_CHAR(object_id, 'xxxxxxxx')) ||'][0x' || trim(TO_CHAR(0,'xxxx')) || '],[SV]' res FROM dba_objects WHERE object_name=upper('&objname') AND owner=upper('&owner') AND object_type LIKE 'SEQUENCE%' / Enter value for objname: TEST_SEQ Enter value for owner: RS RES --------------------------- [0x165d7][0x0],[SV]
Posted in 12c, Oracle database internals, Performance tuning, RAC, weird stuff | Tagged: oracle performance, pluggable database, RAC internals, RAC performance, SV resource, weird stuff | 2 Comments »
Posted by Riyaj Shamsudeen on September 8, 2013
A quick note, Expert Oracle RAC book co-written by me is available now: Expert Oracle RAC 12c. I have written about 6 chapters covering the RAC internals that you may want to learn 🙂 I even managed to discuss the network internals in deep, after all, network is one of the most important component of a RAC cluster.
Posted by Riyaj Shamsudeen on June 12, 2013
This blog entry is to discuss a method to identify the objects inducing higher amount of redo. First,we will establish that redo size increased sharply and then identify the objects generating more redo. Unfortunately, redo size is not tracked at a segment level. However, you can make an educated guess using ‘db block changes’ statistics. But, you must use logminer utility to identify the objects generating more redo scientifically.
Detecting redo size increase
AWR tables (require Diagnostics license) can be accessed to identify the redo size increase. Following query spools the daily rate of redo size. You can easily open the output file redosize.lst in an Excel spreadsheet and graph the data to visualize the redo size change. Use pipe symbol as the delimiter while opening the file in excel spreadsheet.
spool redosize.lst REM You need Diagnostic Pack licence to execute this query! REM Author: Riyaj Shamsudeen col begin_interval_time format a30 set lines 160 pages 1000 col end_interval_time format a30 set colsep '|' alter session set nls_date_format='DD-MON-YYYY'; with redo_sz as ( SELECT sysst.snap_id, sysst.instance_number, begin_interval_time ,end_interval_time , startup_time, VALUE - lag (VALUE) OVER ( PARTITION BY startup_time, sysst.instance_number ORDER BY begin_interval_time, startup_time, sysst.instance_number) stat_value, EXTRACT (DAY FROM (end_interval_time-begin_interval_time))*24*60*60+ EXTRACT (HOUR FROM (end_interval_time-begin_interval_time))*60*60+ EXTRACT (MINUTE FROM (end_interval_time-begin_interval_time))*60+ EXTRACT (SECOND FROM (end_interval_time-begin_interval_time)) DELTA FROM sys.wrh$_sysstat sysst , DBA_HIST_SNAPSHOT snaps WHERE (sysst.dbid, sysst.stat_id) IN ( SELECT dbid, stat_id FROM sys.wrh$_stat_name WHERE stat_name='redo size' ) AND snaps.snap_id = sysst.snap_id AND snaps.dbid =sysst.dbid AND sysst.instance_number=snaps.instance_number and begin_interval_time > sysdate-90 ) select instance_number, to_date(to_char(begin_interval_time,'DD-MON-YYYY'),'DD-MON-YYYY') dt , sum(stat_value) redo1 from redo_sz group by instance_number, to_date(to_char(begin_interval_time,'DD-MON-YYYY'),'DD-MON-YYYY') order by instance_number, 2 / spool off
Visualizing the data will help you to quickly identify any pattern anomalies in redo generation. Here is an example graph created from the excel spreadsheet and see that redo size increased recently.
Posted by Riyaj Shamsudeen on June 5, 2013
The restart of a UNIX server call initialization scripts to start processes and daemons. Every platform has a unique directory structure and follows a method to implement server startup sequence. In Linux platform (prior to Linux 6), initialization scripts are started by calling scripts in the /etc/rcX.d directories, where X denotes the run level of the UNIX server. Typically, Clusterware is started at run level 3. For example, ohasd daemon started by /etc/rc3.d/S96ohasd file by supplying start as an argument. File S96ohasd is linked to /etc/init.d/ohasd.
S96ohasd -> /etc/init.d/ohasd /etc/rc3.d/S96ohasd start # init daemon starting ohasd.
Similarly, a server shutdown will call scripts in rcX.d directories, for example, ohasd is shut down by calling K15ohasd script:
K15ohasd -> /etc/init.d/ohasd /etc/rc3.d/K15ohasd stop #UNIX daemons stopping ohasd
In Summary, server startup will call files matching the pattern of S* in the /etc/rcX.d directories. Calling sequence of the scripts is in the lexical order of script name. For example, S10cscape will be called prior to S96ohasd, as the script S10cscape occurs earlier in the lexical sequence.
Google if you want to learn further about RC startup sequence. Of course, Linux 6 introduces Upstart feature and the mechanism is a little different: http://en.wikipedia.org/wiki/Upstart
That’s not the whole story!
Have you ever thought why the ‘crsctl start crs’ returns immediately? You can guess that Clusterware is started in the background as the command returns to UNIX prompt almost immediately. Executing the crsctl command just modifies the ohasdrun file content to ‘restart’. It doesn’t actually perform the task of starting the clusterware. Daemon init.ohasd reads the ohasdrun file every few seconds and starts the Clusterware if the file content is changed to ‘restart’.
# cat /etc/oracle/scls_scr/oel6rac1/root/ohasdrun
Posted by Riyaj Shamsudeen on August 29, 2012
There are many questions from few of my clients about asmlib support in RHEL6, as they are gearing up to upgrade the database servers to RHEL6. There is a controversy about asmlib support in RHEL6. As usual, I will only discuss technical details in this blog entry.
ASMLIB is applicable only to Linux platform and does not apply to any other platform.
Now, you might ask why bother and why not just use OEL and UK? Well, not every Linux server is used as a database server. In a typical company, there are hundreds of Linux servers and just few percent of those servers are used as Database servers. Linux system administrators prefer to keep one flavor of Linux distribution for management ease and so, asking clients to change the distribution from RHEL to OEL or OEL to RHEL is always not a viable option.
Do you need to use ASMLIB in Linux?
Short answer is No. Long answer is possibly No. ASMLIB is an optional support library and eases the administration of ASM devices. Especially, it is helpful while adding new devices to the nodes in a cluster. ASMLIB essentially stamps the devices and so, it is easily visible in other nodes of a cluster in the next asm scandisk. asmlib also provides device persistence, which is the important benefit of ASM (see the discussion below for more details about device persistence).
Posted by Riyaj Shamsudeen on June 1, 2012
This is a quick note about reverse path filtering and impact of that feature to RAC. I encountered an interesting problem recently with a client and it is worth blogging about it, with a strong hope that it might help one of you in the future.
Environment is 126.96.36.199 GI, Linux 5.6. In a 3 node cluster, Grid Infrastructure (GI) comes up cleanly in just one node, but never comes up in other nodes. If we shutdown GI in first node, we can start the GI in second node with no issues. Meaning, GI can be up in just one node at any time.
System Admins indicated that there are no major changes, only few bug fixes. Seemingly, problem started after those bug fixes. But there were few other changes to the environment /init.ora parameter change etc. So, the problem was not immediately attributable to just OS changes.
Posted in Oracle database internals, Performance tuning, RAC | Tagged: "has Disk HB, advanced RAC training, but no Network HB", cssd not joining cluster, RAC performance, reverse path filtering, rp_filter | 11 Comments »
Posted by Riyaj Shamsudeen on May 22, 2012
Let’s first discuss how RAC traffic works before continuing. Environment for the discussion is: 2 node cluster with 8K database block size, UDP protocol is used for cache fusion. (BTW, UDP and RDS protocols are supported in UNIX platform; whereas Windows uses TCP protocol).
UDP protocol, fragmentation, and assembly
UDP Protocol is an higher level protocol stack, and it is implemented over IP Protocol ( UDP/IP). Cache Fusion uses UDP protocol to send packets over the wire (Exadata uses RDS protocol though).
MTU defines the Maximum Transfer Unit of an IP packet. Let us consider an example of MTU set to 1500 in a network interface. One 8K block transfer can not be performed with just one IP packet as the IP packet size (1500 bytes) is less than 8K. So, one transfer of UDP packet of 8K size is fragmented to 6 IP packets and sent over the wire. In the receiving side, those 6 packets are reassembled to create one UDP buffer of size 8K. After the assembly, that UDP buffer is delivered to an UDP port of a UNIX process. Usually, a foreground process will listen on that port to receive the UDP buffer.
Posted in 11g, Oracle database internals, Performance tuning, Presentations, RAC, video | Tagged: cache fusion mtu, fragmentation and reassembly, gc lost packets, ipfrag_high_thres, ipfrag_low_thres, ipfrag_time, Jumbo frames, MTU, MTU=9000, oracle performance, RAC internals, RAC performance, RAC presentations, RAC training, RAC video, RAC videos, RDS, UDP vs tcp, wireshark | 13 Comments »