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тАО12-24-2003 01:24 AM
тАО12-24-2003 01:24 AM
Re: Kernel Conf for Oracle SGA
Thank you very much for your comment that makes a lot of sense.
For good judgement, I'm going to get HP support to answer it too.
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тАО12-24-2003 01:47 AM
тАО12-24-2003 01:47 AM
Re: Kernel Conf for Oracle SGA
Oracle indeed partitions large SGA into 1Gb segments, so you can setup 3Gb for SGA and you'll see Oracle ask for 3 shared memory segments (probably a way to handle large SGA's on multiple flavors of opsystems). And as mentioned, maxdsiz_64 is just a fence...you can set it to 20Gb if you like. It simply prevents programs asking for more than that amount.
Now, within the 4Gb limit, you can certainly configure Oracle to use 4Gb (or even 8Gb) of RAM for SGA. HP-UX is a virtual memory system and it will gladly accomplish the task as long as there is enough swap space. Of course, there can be a 100:1 performance degradation once SGA exceeds available memory. Hopefully the shock value of that hit will be understood. For a single instance of Oracle in a 4Gb system I would allow 1Gb for the opsystem, buffer cache, memory mapped files and programs, thus allowing abouyt 3Gb for SGA. So the kernel params that need to be changed are:
dbc_max_pct = no more than 400 megs (10% of 4Gb RAM)
dbc_max_pct = 2
maxdsiz=1750 megs
maxdsiz_64=3000 megs
shmmax=2000megs
Note that shmmax=2Gb simply allows a program to request up to that limit. Oracle will request segments in !gb chunks as needed.
NOTE: forget all of this if you are running 32bit Oracle code (64bit HP-UX runs 32bit as well as 64bit programs). There are severe limits for 32bit programs beyond just the 960meg or 1750meg limits which will require careful planning and patches. All 32bit programs must use shared memory from a 32bit pool that all other programs use. That pool cannot be expanded so if you need more, you'll have to (patches!) use memory windows.
Bill Hassell, sysadmin
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тАО12-24-2003 02:44 AM
тАО12-24-2003 02:44 AM
Re: Kernel Conf for Oracle SGA
In my experience - I have had issues with dbc_max_pct greater than 300 Mb with Oracle (specific to Oracle), and have read misc tech notes which recommend that Oracle set the dbc_max_pct to 128 mb
4G x.03 = 120 Mb
So dbc Max should be set to 3 -
Other than that - looks like some real good info here.
Happy Holidays
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тАО12-24-2003 03:22 AM
тАО12-24-2003 03:22 AM
Re: Kernel Conf for Oracle SGA
When DBA's are given a large SGA to play with, one of the first things they will do is to increase the Oracle equivalent of a buffer cache. This one is more effective in that Oracle only adds items that make sense to buffer. HP-UX can't tell the difference between records that will be sequentially accessed (good for read-ahead) and those that are one-time visits. So a small buffer cache (128megs) does make sense with a large SGA, but you'll need Online JFS to add the options to the Oracle data disks (only) to bypass the buffer cache and use Oracle's buffering exclusively.
Bill Hassell, sysadmin
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тАО12-26-2003 12:06 AM
тАО12-26-2003 12:06 AM
Re: Kernel Conf for Oracle SGA
However, it is always good to follow rules of thumb like those discussed by others previously (e.g., don't have SGA exceed free memory, reduce DBC size if Oracle has adequate buffers, etc.), just consider some of the more involved changes (e.g., Oracle bypassing HP-UX buffer cache) more seriously before implementing only if you've identified a specific bottleneck that the change might address.
And of course the NUMBER 1 RULE of performance tuning. Make only one change at a time...
=;-) Alex
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