Re: Multiblock_read_count setting

Dave Morgan wrote:

> Hello all,
> I understand that the multi_block read_count parameter is
> used when doing full table scans. However, I do not understand
> which blocks are being refered to (db, os or physical disk) or
> what the setting should be.

they are db blocks ( and luckily your oracle block happens to match a standardUFS block on Solaris @ 8K

>
>
> Our current databases are:
>
> Oracle 7.2.2.4 on Solaris 2.5.1

its a good bet that you max_io_sz ( db_block* multi_block_read_count )is 128K - so if you have a look at v$parameter
SQLPLUS> select * from v$parameter;
and have a look at multi_block_read_count you will find that it is automatically throttled back to 16 - regardless of what init.ora says ...this is because oracle goes out and tests a device to see how large an IO can be issued

some /etc/system params that control amount of IO sent to a filesystem file in Solaris 2.5.1 are:

ufs:ufs_LW - per file IO outstanding which will allow read-ahead ( in Solaris UFS ) ( def 256k)
ufs:ufs_HW - per file IO outstanding which waits for this files buffers to drain to LW before

    reading ahead ( def 384K)
maxphys - amt of physio ( how much the device(host bus adapter and scsi disk can handle))
 ( all of above are set in bytes and are set in /etc/system requiring a reboot to take effect )
you should also make sure that when you newfs(1) your filesystem if you are using UFS
that you change the 'maxcontig' parameter as this control UFS clsutering ( read-ahead,write-behind)
and once Solaris issues an IO this is how big a SCSI cmd block is built and sent to the SCSI
subsystem for UFS IO.

Solaris 2.5.1 will allow very large IO's to be issued to raw devices ( up to 1MB ) via Sun's Kernel Asynch IO feature as long as you set the maxphys parameter in /etc/system to 1048576 and use Oracle 7.3

hth
chris

> No RAID, no mirroring just standard SCSI
> Database block size = 8kb
> database_block_buffers = 14000
>
>
> The settings I inherited for the multiblock_read _count
> range from 4 to 256.
>
> Any suggestions, answers or ideas welcomed.
>
> Dave
>
> / Dave Morgan, Database Administrator
> / QC Data Petroleum Services Ltd.
> / E-Mail: dave_morgan_at_calgary.qcdata.com
> / http://www.qcdata.com
> /
> /I am allowed to have opinions. If my wife approves of them I am allowed
> /to have them.
Received on Tue May 26 1998 - 22:09:08 CDT