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Q and A
Asked and Answered
Ever wonder what prefetch size you should use with RAID-5?
I have a question about RAID-5. I was advised to calculate the
extentsize as 6*64k/4k [equal] 96 (RAID-5 is 6+1+1 disks). (64K is stipe
size of RAID.) Since there is only one container per RAID-5, should I
use
prefetch size 96 or 96*6[equal] 384?"
Sincerely,
Holo Devnani
Toronto, Canada
Scott Hayes responds:
Questions about RAID come up frequently; I hope I can help clarify what
to do with DB2 UDB in a RAID environment.
First, before creating your tablespaces, use the
db2set
command to set
the environment variable
DB2_STRIPED_CONTAINERS=ON
. By doing so, you
will reduce physical I/O by as much as 50 percent.
For tablespaces that are created on RAID devices, tablespace IDs must be
either listed in environment variable
DB2_PARALLEL_IO
, or use an
asterisk (*) to indicate all tablespaces are defined on RAID devices.
You can find the Tablespace ID by using the command
db2 list
tablespaces
. If tablespaces 3,4,5,6, and 7 are on RAID, the proper
command to issue would be
db2set DB2_PARALLEL_IO=3,4,5,6,7
.
By setting
DB2_PARALLEL_IO
, the tablespace PREFETCHSIZE takes on special
meaning. PREFETCHSIZE is divided by the EXTENTSIZE to arrive at the
degree of I/O parallelism. Without this environment variable set, the
degree of I/O parallelism is normally derived from the number of
containers. Because RAID often has only one container, it is important
to set the PREFETCHSIZE as a multiple of the EXTENTSIZE, to provide a
sufficient number of IO_SERVERS (at least one per physical disk), and to
assign the
tablespace to a bufferpool that is sufficiently large to
accommodate to prefetch requests.
If prefetch I/O is not desired because the database supports an OLTP
application, then do not include the tablespace ID number in the
DB2_PARALLEL_IO
environment variable definition, or set PREFETCHSIZE
equal to EXTENTSIZE.
See a
complete archive of reader/author Q&As
.
Back to
Top 10 Performance Tips
by Scott Hayes.
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