OS Buffer Size

[Andrew Porter_1]

I am running oracle on this hp box and oracle support asked me to check on the "OS Buffer Size" does any one know where kernel parm that is?

[Jean-Louis Phelix]

Hi,

Perhaps they are talking about buffer cache. They often recommend to set it low (dbc_max_pct ~ 10 and dbc_min_pct ~5)

Regards,

Jean-Louis.

[John Poff]

Hi,

My guess is that they are referring to the buffer cache, which is controlled by the dbc_max_pct and dbc_min_pct kernel parameters. The default out of the box is for the max to be 50%, which means it will use up to 50% of available RAM, which is way too much. Most of us run around 5% for the min and 10% for the max, trying to keep the total buffer cache size around 400Mb to 800Mb. You have to tune the kernel and reboot to change it.

You can search the forum and find a bunch of threads discussing buffer cache.

JP

[Paula J Frazer-Campbell]

Andrew

dbc_max_pct and dbc_min_pct are not set to specific figures, but are based on how much memory your system has installed.

The default settings of max=50% and Min=5% can be incorrect but is dependant upon memory.

Targets are Circa 400 meg for Max and Circa 80 meg for Min.

So if you have 4 Gig of memory then:-

Max =

400/4000 x 100/1 = 10%

Min=

80/4000 x 100/1 = 2%

An adjustment requires a reboot and monitoring can be done by :-

sar ???b

and watch the %rcache column for figures below 100% -

That is:- the system was not 100% successful in finding what it wanted in the cache and so therefore user the slower medium of disk.


05:00:00 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
05:01:00 243 15477 98 4 128 97 0 318
05:02:00 172 11227 98 6 170 97 0 279
05:03:00 89 21358 100 5 123 96 0 284
05:04:00 258 19605 99 7 121 94 0 332
05:05:00 267 9727 97 2 136 98 0 345
05:06:00 150 10813 99 3 172 98 0 286
05:07:00 326 7702 96 5 218 98 0 302
05:08:00 297 13572 98 3 178 99 0 314
05:09:00 391 5470 93 6 83 93 0 282
05:10:00 413 3306 87 4 57 94 0 326
05:11:00 374 3620 90 3 61 95 0 287
05:12:00 215 3986 95 4 82 95 0 205
05:13:00 401 3264 88 2 59 97 0


HTH

Paula

[Andrew Porter_1]

I am not sure this is the correct parm.

Oracle is speaking of reading from disk, IO....

So I am incline to beleive that is has some thing to do with the max read size from disk per read....

Any ideas

[James Murtagh]

Hi Andrew,

I agree with the last two replies that they are requesting the filesystem buffer size. However, to ensure you are using a dynamic buffer cache (sized by dbc_min_pct and dbc_max_pct) you must have the static buffer parameters (nbuf and bufpages) set to 0 in the kernel. Although the dynamic buffer is the default I still see a few systems using the fixed cache.

Regards,

James.

[T G Manikandan]

It depends on the amount of memory and the amount of memory that your system uses.

The dbc_max_pct and min_pct are the parameters which give a range for the buffer cache.

It is not fixed.It is dynamic.
So by default it ranges from 5% to 50%.

Oracle in turn has a database buffer cache.
So the parameter can be reduced to 15% if your memory is 2Gb.
I would go with the 300MB of maximum buffer cache on the system.
check this link for more info in the kernel parameters

http://docs.hp.com/hpux/onlinedocs/939/KCParms/KCparam.DBCmaxPct.html

[Ian Lochray]

Andrew,
are you sure they don't mean the OS block size?

[Andrew Porter_1]

Oracle's ability to read multiple blocks is limited by the operating system's upper limit on the number of bytes which can be read in a single I/O call (MAX_IO_SIZE).

Does the parms metioned above control this or is this something else?

[Ian Lochray]

From HP's documentation ...

What is the maximum single I/O transfer size?


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date: 8/17/00
document description: What is the maximum single I/O transfer size?
document id: KBRC00003661





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--------------------------------------------------------------------------------
What is the maximum single I/O transfer size? DocId: KBRC00003661 Updated: 10/1/02 10:02:00 AM

PROBLEM
To tune Oracle it is required to know the maximum io transfer size. What is the maximum single I/O transfer size on a HP-UX system?
RESOLUTION
1) Using raw disk devices, the maximum I/O transfer size is 1 megabyte.

2) Using LVM (Logical Volume Manager), the maximum I/O transfer size is 256
kbytes.

3) Using LVM with LVM striping, the maximum I/O transfer size is limited to the
size of your strip or at most 256k.


The disk io drivers will also try to combine the requests, so it's possible to
have a larger I/O transfer size, but since this is not easily calculated, there
is no way to know what the highest io transfer size could be.
ALT KEYWORDS
max_io_size tablespace ORA-3232 max_io_sizeKEYWORDSmax_io_sizetablespace (ORA-3232)