21. DB2 UDB and the 2.6 kernelDB2 UDB Version 8.2 can take advantage of a number of performance
enhancements introduced in the 2.6 kernel. There is a good paper by Rav
Ahuja, Dan Behman and John Keenleyside that goes in depth on the
specific enhancements of DB2 UDB V8.2 (see Resources); we'll
cover a few of the highlights and how to implement them.
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Note that these enhancements are for advanced users only. Implement
each with care and be sure to thorougly test each change individually
before moving on to the next enhancement.
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Asynchronous I/O handling Asynchronous I/O is a kernel processing
enhancement that allows applications to submit I/O requests, and then
continue working without having to wait for the I/O request to be
completed. As mentioned in a few of the
distribution-specific instructions, this requires the installation of
the libaio package, and the setting of the DB2LINUXAIO DB2 UDB
configuration parameter. Direct I/O caching Direct I/O is a different cache writing policy. Instead of using buffered I/O
writes, direct I/O caching eliminates the file copying from the file
cache to user buffer, thus reducing CPU utilization. To enable Direct I/O cache on a database, you specify the NO FILE SYSTEM
CACHING parameter for the CREATE, TABLESPACE, ALTER TABLESPACE or CREATE
DATABASE commands. Vector I/O Vector (or scatter/gather) I/O allows a number of I/O buffers to be used instead of
one contiguous buffer, thus avoiding additional copying
during reads. To enable Vector I/O, set the DB2_SCATTERED_IO
configuration parameter to "ON". Large page support Large page
support allows the application to force the kernel into using larger
memory pages, thus reducing the memory overhead required to track
the pages, boosting the overall available memory. Enabling
large page support requires both a 2.6 kernel (or a backported 2.4
kernel like the one in RHEL), and installation of the libcap RPM.
Turning on large page support is distribution dependent. For
SuSE Linux Enterprise 8:
During boot-up, add the parameter bigpages=xxxM to the
kernel boot command, where xxx is the desired large page size in
MB After boot, log in as root and run
echo 1 > /proc/sys/kernel/shm-use-bigpages |
This
will dynamically enable the large page support in the
kernelBefore running db2start, execute this command to tell
DB2 UDB to use the large page configuration:
For
Red Hat Enterprise Linux 3
As root, run:
echo XXX > /proc/sys/vm/hugetlb_pool |
where XXX is
a numerical value equivalent to the number of "huge pages" desired. (Red
Hat Linux uses "huge" pages instead of large pages). Note also that
the size you enter must be available in a contiguous block of memory,
otherwise the large page limit will be restricted by the largest block
that is available.Check the large page usage by running the command:
cat /proc/meminfo | grep -i huge |
Before running db2start, execute this command to tell
DB2 UDB to use the large page configuration:
For
Distributions using 2.6 kernel
As root, run:
echo XXX > /proc/sys/vm/nr_hugepages |
where XXX is
a numerical value equivalent to the number of "huge pages"
desiredCheck the large page usage by running the command:
cat /proc/meminfo | grep -i huge |
Before running db2start, execute this command to tell
DB2 UDB to use the large page configuration:
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