Re: iostat - multiblock read count

utkanbir wrote:
>
> My system is a two node rac on redhat linux 2.1 , 4 ia64 cpus ,
> 8gb.ram , ocfs and emc raid 10

time to move to RH3? ;)

> The db_block_size is 16kb. , current multi_block_read_Count is 64
> (which makes 1mb. of read)

can mean nothing in Linux, read on...

>
> 1.It seems when i decrease the multiblock read count parameter , the
> rsec/s increases , but i expect to see the opposite.What's wrong with
> this?

I've got a funny feeling you just hit the 32K default Linux I/O limit. You see, until kernel release 2.6 (or patched 2.4), Linux will "secretly" transform any single I/O request for more than 32K bytes into as many 32K requests as needed. This takes time and physical overhead from the disk controller(s). When you reduce the dbfmr, you reduce this overhead and paradoxically (my my, what a long word for "D'uh!"...) you end up with a little more r/s. Read on.

> 3. Related to my second question , since i use raid 10 (striping +
> mirroring ) is it possible to get higher io rates ? Looking at the
> iostat values , (rsec/s) / (r/s) more or less equals to 28kb. (This
is
> the kb. read in each read.)It seems a very low value to me. On a sun
> solaris ufs file system for instance , i can achive 128KB or even
1MB.
> per read by playing with the parameters , i dont understand why my
> linux box is different.

There is a patch for RHAS at Oracle Metalink that gets rid of this 32K limitation. It applies AFAIK only to 2.4.21 onwards, until RHAS4 whereupon the 2.6 kernel takes over and it's not a problem anymore. However I'm not sure if this patch is compatible with anything under Oracle 10g, so CHECK first with support.

If this is your problem you need first to upgrade to the adequate level of RHAS3, *then* apply the Oracle patch. You'll need to request it from Oracle support themselves.

Go here:
http://www.oracle.com/technology/deploy/availability/pdf/ora_lcs.pdf for all the nasty details.
HTH Received on Mon May 23 2005 - 02:44:36 CDT