Oracle Linux Debugging and Configuration Best Practices

Debugging and Configuration
Best Practices for
Oracle Linux

Greg Marsden
Senior Director, Linux and Virtualization


Agenda

 Key Linux Tips and Tricks
 Common Issues
 Diagnostic Tools and Use Cases
 Do it Yourself Debugging
 Ksplice in the Datacenter


Tips and Tricks:
Key Points


Key Linux Tips and Tricks

Kernel Tuning: Oracle
Preinstall RPM
Best Performance
Diagnostic Tools: and Reliability
kdump and oswatcher


Oracle Preinstall Package and Templates
Configure Oracle Products Automatically

 oracle-rdbms-server-11gR2-preinstall-1.0.6.el6.x86_64.rpm
 Per-Product Preconfiguration Package
– Based on Validated Configuration‟s real world stack testing
– Includes Product Release Notes recommendations
– Installs necessary dependencies and kernel tuning parameters
– Individual for each Oracle product
 Oracle VM Template for Oracle RDBMS Server
– Production-ready, installed virtual machine templates from eDelivery


System Diagnostics
Critical Diagnostics Software should run at all times

 oswatcher utility: Install and leave running to collect over-time
information about system activity.
 serial console or netconsole to remotely monitor system activity in
the case of a disk, network or system outage.
 kexec crash collection utilities to gather forensic information from
malfunctioning systems.


Tips and Tricks:
Memory Management


“Help! my system has 250 GB of RAM I‟m running out of
memory! My consultants are telling me we can‟t scale
with a 120GB SGA and this many connections, but I
can‟t fit any more RAM in this system.”
Anonymous DBA
Oracle User


Issue: Not using Hugepages

Frequent Issue I found the following:
.
• Symptom: Out of Memory Errors, slow 13:09:19 57591060k free 159 client connections
performance. Detected via 13:26:01 26189944k free 1826 client connections
oswatcher. 13:32:31 15547144k free 2024 client connections
13:57:00 467048k free 2037 client connections
• Cause: SGA mapped in 4k pages (here is where we begin swapping memory to disk)
instead of 2MB
I also found this:
• Solution: Use Hugepages
zzz ***Fri Aug 9 13:23:22 PDT 2011
• Hugepages are faster.
MemTotal: 250 GB
• Hugepages are “pinned” and won‟t MemFree: 464 MB
be swapped. PageTables: 112 GB


Performance with Hugepages

Without Hugepages With Hugepages
• 200 Connections to a 12.9GB SGA • 200 Connections to a 12.9GB SGA
• Before DB Startup Pagetables: 7400 kB • Before DB Startup PageTables: 7748 kB
• After DB Startup Pagetables: 652900 kB • After DB Startup Pagetables: 21288 kB
• After 200 PQ Slave run query • After 200 PQ slaves run query
Pagetables: 6189248k Pagetables: 80564 kB
• Time to complete: 00:10:23.60 • Time to complete: 00:00:18.77


Hugepages and Transparent Hugepages
Performance for DB and Middleware Applications

 “Regular” Hugepages [Ref. Doc ID 749851.1]
– Reduce footprint of individual Oracle database connections.
– Increase performance and scalability.
– Requires manual tuning after SGA changes, and does not work with AMM.
 Transparent Hugepages
– Transparent hugepages do not help the RDBMS use case.
– Auto-allocate hugepages for large memory allocations. Great for
Java/middleware/applications.
– New for UEK and OL6!


Issue: Slow Performance at 95% RAM
OL5 Specific issue with large memory allocation

 Symptoms
– System is swapping and shows low free/cached memory
– Reduced system performance
 Cause: Usually the kernel is hogging CPU in try_to_free_pages from
pagecache, inactive lists.
 Solution
– Ensure you are running a shrink_zone patched kernel: UEK, OL6, or
OL5+BUG6086839
– If system is swapping but performance is OK, get more RAM.


Issue: System Swapping with Free Memory
NUMA Specific Problem

 Symptom: System starts to swap while reporting free RAM
– vmstat reports free memory.
– dmesg has “order 5 allocation failed” messages.
– If <5 order allocations are failing, there are larger issues
 Cause: Memory Fragmentation. On NUMA systems, caused by
fragmentation of node-local memory for kernel applications.
 Solutions:
– Disable NUMA
– Decrease MTU size if using jumboframes


Memory Accounting
What is Linux Doing With My Memory?

 Free = Cached + Free
– All free space on Linux is used for pagecache.
– This behavior cannot be disabled.
 Process Shared Memory is hard to find in Linux
– RSS double counts shared memory, Total includes unmapped pages.
– Use /proc/<pid>/smaps to see real process memory usage.
 cgroups: New features in the latest kernels let you restrict RAM
– Useful to throttle pagecache use by backup processes


Swap: What is it good for?

Tuning Swap Space
 Swap is a highly contentious topic on
Linux
– Benefit: Allows “room to grow” for
inadequately sized systems.
– Drawback: Much slower than memory
access, often makes problems worse.
 Recommendation: Use swap, but
ensure IO to swap disk is kept close to
zero.

vmstat output


Tips and Tricks:
General Recommendations


Other Configuration Trouble
Assorted Common Configuration Issues

 Use UUID to Mount System Disks
– Symptom: System panics after upgrade
– Cause: New hardware, drivers, or kernel reorders device discovery
– Cautions: May not work with LVM snapshot
 NFS Locks Not Released on Reboot
– Cause: kernel and DNS have different hostnames
– Solution: ensure kernel hostname is fully qualified. See BUG 3156942.
 Cluster Reboots with OCFS2
– Cause: Network or Disk outages can cause OCFS2 to fence nodes
– Solution: Ensure OCFS2 timeouts are greater than storage/network failover
timeouts. Defaults may be too short for o2cb heartbeat.


Performance Tuning Kernel Parameters
System Scheduling
 vm.swappiness
 100: Force aggressive swapping
 0: Insurance against a backup process hogging all system memory
Network Protocol Buffers
 net.core.wmem_default/max: Buffer size for outgoing network packets.
 net.core.rmem_default/max: If these values are set too small, system may discard TCP packets

Memory Management
 vm.dirty_ratio: encourage frequent pagecache writeback
 vm.lowmem_reserve_ratio/vm.min_free_kbytes: reserve physical memory for kernel allocations


Bugs Fixed in Enterprise Linux

Oracle Linux 5 Bug Fixes

 Oracle finds and fixes critical
bugs in Enterprise Linux.
– Red Hat Compatible Kernel vs.
Oracle-Modified kernel
– Install the Compatible Kernel for
bug-for-bug compatibility with RHEL

 Patches required for correct
Oracle product operation


Oracle Patches Linux
Specially Tuned Linux Kernels for Customer Requirements

 Staying up-to-date with your Linux distribution is very important.
 Bug-Fixed Oracle Linux Kernel
 UEK: Unbreakable Enterprise Kernel
– Top Performing Kernel. World Record TPCC Benchmark.
– Provides OL6 performance on OL5 systems.
 Backporting of fixes is a temporary solution, not a permanent one.
– Always plan to update or ksplice to the latest kernel version.


UEK: Modern Linux for Oracle

Fast, Modern, Reliable
 Get the latest in performance and
features from Linux, tested by Oracle.
– All new kernel, optimized for Oracle.
 Stay closer to mainline Linux with
patches to improve performance for
Oracle workloads.
– All patches are open source and
submitted to mainline Linux
– Patches provided via RPM and via ksplice
– World Record TPCC Bencmark March
‟12.


Tips and Tricks:
Diagnostics


Issue: Post-Event Diagnostics
What to do after a crash or hang?

 Hard hangs
– Panic, OOPS, nmi_watchdog
– “Spontaneous Reboot”
 Brownouts
– Performance Degredation

 Cluster Scenarios
– Network or Disk may have gone away, triggering the fence
– Need to maintain crash data in the event of loss of net/disk
– Ensure timeouts (like OCFS2) are set correctly


Two Kinds of Critical OS Logging

Continuous OS Logging Panic/Hang Event Logging
 oswatcher continuous logging collected  serial console or netconsole should always
timestamped snapshots of system commands: be set up for any production system. No
 ps, top exceptions.

 slabinfo, meminfo  Consoles also preserve sysrq data.

 vmstat, mpstat, iostat  kdump system memory image collection.

 Other tools can be employed as well, like sar
or collectl.


Console Logs
Finding Faults if Disk or Networking Fail
 Kernel Messages may not be available after a crash
– Serial Consoles are proven technology for preserving console output
 How to capture Serial output:
– Reliable ways to capture serial output:
 ILOM virtual console
 Serial-Over-Lan BIOS config
 Inexpensive DB9-USB converter or Serial Concentrator
– Unreliable ways to capture serial output:
 Physically attached terminal with „setterm –blank 0‟ and system not configured to reboot
 netconsole (can be difficult to configure, and subject to network outages)
 Things to check:
– Ensure Baud Rate is high enough (not 9600 baud)
– For Virtual Console, ensure console history is setup to capture large amount of output


Keyboard and Console Diagnostics
SysRq Key Combinations for Diagnostics
Magic SysRq Key How to Invoke Magic SysRq…
M: dump system memory statistics Console:
Alt + Sys Rq + <cmd>
P, W: dump the stack for all processors Serial Console:
<Break> <cmd>
T: dump the kernel stack trace for all processes Command Line:
echo t > /proc/sysrq-trigger
C: Immediately cause a system crash Oracle VM dom0:
xm sysrq <cmd> <domain ID>
S … U… B: Emergency Sync all disks, Unmount disks, Ensure kernel.sysrq = 1
reboot.
Some of these operations (like stack trace) dump a lot of These operations take full priority in the kernel. Do
data (1MB or more!). not run them in your monitoring scripts, use
carefully!


Diagnostic/Destructive Kernel Parameters
Enable Keyboard- and Console-based Debugging
 kernel.sysrq=1
 Always have this set to enable debug commands
System-wide Events
 panic_on_oom: Panic for Out of Memory condition
 (Alternative would be to kill the high memory process)
 panic_on_oops: Panic for system problems
 (off: some modules may survive a panic, but system state is inconsistent)
Per-Process Events
 hung_task_timeout: Enable warning if process not scheduled for (timeout) second.
 Can cause a lot of log messages, not usually useful
 hung_task_panic: Cause a stack trace and system panic if the timeout is hit
 Can be useful for debugging. Not good to set by default.


Crash Kernel Memory Snapshots
Set Your System to Automatically Dump Core

 kdump: uses Linux kexec function to save kernel stacks after a panic
– Only way to get diagnostic data if disk or network are not available.
– Reboots the system into a protected memory area to save crashed kernel
 Very common errors:
– Not testing kdump: Requires specific memory tuning (crashkernel=) and
also requires specific HBA or network drivers
– Have dedicated space for crash dumps. Preferably not in your root
partition. Remember, vmcore == physical memory size.
– Local disk is faster and more reliable than network dumps.
– Use gzip or `makedumpfile` to compress cores prior to upload


Reading a Kernel Core

Using the Crash Utility

 Crash `bt` backtrace or SysRq-T stack
– Get debug symbols from oss.oracle.com

 Red flags:
– Many processes in D state (IO)
– Many processes in same kernel routine
(contention?)
 Caution: Stack traces can be 1M or
greater. Don‟t do this frequently.

dmesg output after SysRq-T


Diagnostic Tools for Brownouts
Getting more out of your diagnostic tools

 strace -ttT: Diagnose slow processes
– Automatically timestamp system calls
– Useful for diagnosing specific process syscall latency
– Also helpful to determine if a problem is in kernel or usermode
 Crash utility on Virtual machines
– `xm dump-core` takes a noninvasive kernel snapshot of a system
– Provides memory, stack traces, and kernel logs


Tips and Tricks:
Ksplice


Issue: Old Kernels
Newer Kernel Releases Fix Your Bugs

 Symptom: Customer systems are encountering known/fixed issues
– Examples: tcp window_size, shrink_zone, etc.
– Use new kernels for new features: NFSv4, dtrace, btrfs.
 Cause: Older kernels are not „stable‟. New kernels fix bugs.
 Solutions:
– Implement a periodic update schedule for kernel and OS packages, or…
– Use ksplice to stay up to date with patches


Ksplice: Rebootless Kernel Patching
Zero Downtime Patching for Bugs and Security Updates

 Ksplice keeps your system up to date
– Integrated with ULN
– Now available in online and offline modes
 Using Ksplice for Diagnostics and Patching
– Real-World NFS Example


Summary

 Key Linux Tips and Tricks
– kexec and oswatcher

 Common Issues
– memory management and configuration

 Diagnostic Tools and Use Cases
 Ksplice in the Datacenter


ORACLE LINUX PAVILION

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Oracle Linux Sessions
Tuesday, Oct 2nd

Oracle Linux TRACK SESSIONS

General Session: Oracle Linux Strategy
and Roadmap
GEN8726 10:15 AM Moscone South - 103
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Top Technical Tips for Automatic and
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CON8731 Speakers: Lenz Grimmer, Oracle, Martin 11:45 AM Moscone South - 270
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Wednesday, Oct 3rd
Oracle Linux TRACK SESSION

Why Switch to Oracle Linux?
Moscone South -
CON8729 Speakers: Monica Kumar, Mike 3:30 PM
270
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HANDS ON LABS
HOL9383 Oracle Linux Package 10:15 AM Marriot Salon
Management: Configuring and 14/15 YB level
Enabling Services
HOL9384 Oracle Linux Storage 11:45 AM Marriot Salon
Management with LVM and 14/15 YB level
Device-Mapper


Thursday, Oct 4th

HANDS ON LABS

Oracle Linux Package
Marriot Salon 14/15
HOL9383 Management: Configuring and 12:45 PM
YB level
Enabling Services
Oracle Linux Storage
Marriot Salon 14/15
HOL9384 Management with LVM and Device- 2:15 PM
YB level
Mapper


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Resources
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@ORCL_Linux Facebook.com/ Blogs.oracle.com Oracle Linux YouTube.com/
OracleLinux /linux Experts Group oraclelinuxchannel

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