ITEM: I6287L

Ordering my disks on 2 controllers for maximum performance.

Question:

I have 6 disk drives, all of which are a part of rootvg. I have a very
large database in rootvg also. I am going to move 3 of the disks to
another SCSI controller. I am using disk striping as well. My question
is which drives should I put on each controller. I figured I should
put hdisk0, 2, and 4 on one and 1, 3, and 5 on then other. If I am
doing sequential reads and writes, will these be the best order?  I
want to know how to order my disks for maximum performance.
Response:

I told you that the placement of the disk drives did not really
matter.  It was the placement of the Logical Volumes (LV) that would 
do the striping you need.  I told you that you could use the 
extra-policy of "maximum" to spread a LV across a selection of drives 
upon creation of the LV.  This will probably achieve the kind of 
"striping" you want. If not, you can specify a map for the creation 
of the LV that will specify where EXACTLY on the hard drive to place 
the LV (ie- which physical partitions (PPs) it would occupy).

You also wanted to know why we recommend that rootvg be kept separate
from the data. There are many reasons but the primary reason is that
in the event of a system crash, it takes considerably less time to
reload the rootvg from a mksysb. All you have to do after the install
is to import the other volume groups, and the system is back up. Also, 
a mksysb does not make backups of unmounted filesystems or RAW LVs. 
Keeping these in another volume group makes them easier to manage and
you lower the risk of losing data if the operating system goes down
and you have to reinstall.

If you use all the disks for rootvg, you could keep the Operating
System (OS) LVs on one disk.  If the OS LVs are on other disks, you
can migrate all of them to fit on one disk, if the disk is big enough. 
You will need to keep in mind the special procedures that might be 
needed for the moving of hd5 and hd7.



Support Line: Ordering my disks on 2 controllers for maximum performance. ITEM: I6287L

Dated: April 1994 Category: N/A

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