New Oracle tricks inside
Redhat Linux release 5
Article by author Mladen Gogala
1) CFS and ionice
Kernels
newer then 2.6.13 come with the "CFS" (Completely Fair
Scheduler) I/O scheduler which can assign I/O priorities to
processes.
ionice -c 1 -n 0 -p `pgrep -f
lgwr`
will assign the highest real-time priority to the oracle log
writer. If two or more processes have something to write to the
same disk, log writer will always get there first. The same
thing can be done with DBWR as well.
The "ionice" command helps you prioritize I/O done by
processes. The class 1 is "real time", the class 3 is "idle".
Process in class 1 with the priority 0 has the highest I/O
priority, any other process will have to wait for the LGWR to
complete. If LGWR is the only process doing I/O on that
particular disk, there will be no gain.
There isn't much
sense in doing that when ASM is involved, as ASM usually has
its own piece of disk for exclusive use. This usually helps on a
busy file system single instance databases, where file systems
are used for other apps as well. Classic example is having both
Apache and Oracle on the same system. Those two pieces of
software have completely different I/O usage patterns and you
may decide to give one of them higher priority than the other
one. It doesn't necessarily have to be Oracle.
2) VDSO (Virtual Dynamic Shared Object)
Thanks to the Intel Corp., the newer kernels have the new
system call entry/exit mechanism called "VDSO". This mechanism
is about 40 CPU cycles cheaper then the old, interrupt based
mechanism.
This is 40 CPU cycles per system call invocation, which can
accumulate quite a lot. Admittedly, the effects are not
spectacular but help squeeze out every last drop of the CPU
power you have. In order to enable it, one can either enable it
through sysctl or do something like:
echo
1> /proc/sys/vm/vdso_enabled
Newer
kernels come with that enabled, so there isn't much to do.
3) Blockdev
The "blockdev"
command is used to get/set disk characteristics at the driver
level. One particularly useful option is to set readahead to,
say, 16M. I've never done this with a production instance but
the effects were good on an Oracle 10.2.0.4 running on top of F8.
Direct I/O was not turned on, testing is in progress.
4) Readahead.
Redhat EL 5.x has
two services called "readahead_early" and "readahead_later".
Both services will do fstat of the files listed in the
configuration directory (/etc/readahead.d/default.*) and get the
corresponding inodes in the inode cache, thus speeding up the
next open. It's quite a nice thing to do with oracle: All files
in $ORACLE_HOME/bin, $ORACLE_HOME/lib and various "*.jar" files
can all be cached to speed up later reads and activation.
These are few dirty tricks possible mainly with the RH EL
5.x and its clones (CentOS, OEL). Experiment but be aware that
this is a new and largely untested stuff, unsupported by Oracle
Corp. and probably not fit for production.
There will be more good stuff to come. with the kernels
2.6.20 and newer. The main thing with 2.6.20 is that it does I/O
accounting, so that the question "who does the most of disk I/O"
can be answered. There is a simple tool called "iotop" which
works on kernels
2.6.20+ and does just that.
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