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Improper Disk Setup

Oracle Tips by Mike Ault

Under the heading of improper disk setup there are many sub topics. Some of these disk setup topics include:

·         Interface issues

·         Mount options

·         Filesystem choices

·         RAID setup

·         Disk size and speed choices

Let’s look at each of these in the Oracle environment.

Interface Issues

Generally interface issues resolve to bandwidth issues. A case in point, a major bakery had upgraded their system, putting in more, faster CPUs, higher speed disks and newer hardware overall. They calculated on the average they only used 75% of the bandwidth on the old system so they reduced the number of HBAs from 12 dual-channel to 8 dual-channel.

After the upgrade performance looked great, until the end of month processing crunch, suddenly performance dropped to half of what it was before. Investigation showed that while on the average they only needed 75% of the bandwidth of the 12 HBAs during end of month, end of quarter and end of year processing they actually required more. Luckily for them the HBAs in the old system where compatible and, they had the needed expansion slots to add the needed HBAs to the new system. With the 4 additional HBAs in place they quadrupled their performance.

The other major choice in interfaces is in interface type, SCSI, Fibre, Fabric. Unfortunately there is no simple answer, you need to examine your system and if IO timing is bad, find out whether it is related to contention or bandwidth issues.

Note that when you monitor IO timing you need to look at it from Oracle’s perspective, that is, from the time Oracle requests the IO to the time the IO is received by the Oracle system, not strictly at the operating system level. If you see a large difference between what Oracle is saying IO timings are and what the OS is saying you need to track down where the time is being consumed between the disk packs and the Oracle database.

Mount Options

Essentially Oracle doesn’t like any mount option involving logging on its datafile mount points. Anything you can do to make the disk look RAW to Oracle is a good thing. This means nologging, noatime, async, aio and many other mount options need to be considered depending on your operating system. Dramatic improvements in performance have been reported from just changing the mount options on the drives otr filesystems that support your Oracle datafiles. Generally it is not advised to switch to nologging type options on filesystems where non-Oracle or binary type files (executables) are stored.

In UNIX you can control whether a file system uses buffered or unbuffered IO. With Oracle the use of a buffered filesystem is redundant and dangerous. An example of the dangers of a buffered filesystem with Oracle is when power is lost. The buffer in a buffered filesystem depends on the cache battery to provide enough power to allow the buffer to be written to disk before the disk spins down. However, many shops fail to monitor the cache battery lifetime limitations or fail to change the batteries at all. This can result in loss of data in a buffered filesystem on loss of power.

You can turn off buffered writes in several ways (buffered reads aren’t an issue, but you should always use write-through caching). One is to mount the filesystems used with Oracle files as non-buffered using such options as:

AIX: “dio”, “rbrw”, “nointegrity” 

SUN: “delaylog”, “mincache=direct”, “convosync=direct” ,”nodatainlog”

LINUX: “async”, “noatime”

HP: Use VxFS with: “delaylog”, “nodatainlog”, “mincache=direct”, “convosync=direct”

Using Direct IO at the Oracle Level

For information about Oracle direct I/O, refer to this URL by Steve Adams:

* http://www.ixora.com.au/notes/filesystemio_options.htm

Checking Your Server

Methods for configuring the OS will vary depending on the operating system and file system in use. Here are some examples of quick checks that anyone can perform to ensure that you are using direct I/O: 

·         Solaris - Look for a "forcedirectio" option.  Oracle Remote DBAs find this option often makes a huge difference in I/O speed for Sun servers.  Here is the Sun documentation:  http://docs.sun.com/db/doc/816-0211/6m6nc6713?a=view

·         AIX - Look for a "dio" option.  Here is a great link for AIX direct I/O: 

http://www-106.ibm.com/developerworks/eserver/articles/DirectIO.html

·         Veritas VxFS - (including HP-UX, Solaris and AIX), look for "convosync=direct".  It is also possible to enable direct I/O on a   per-file basis using Veritas QIO; refer to the "qiostat" command and corresponding man page for hints.  For HPUX, see Oracle on HP-UX – Best Practices

·         Linux - Linux systems support direct I/O on a per-filehandle basis (which is much more flexible), and I believe Oracle enables this feature automatically.  Someone should verify at what release Oracle started to support this feature (it is called O_DIRECT). See Kernel Asynchronous I/O (AIO) Support for Linux  and this great OTN article: Talking Linux: OCFS Update.

SEE CODE DEPOT FOR FULL SCRIPTS


For more information on this topic, I recommend Don Burleson's latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

 


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