Re: Paging Space Placement Question

steven_nospam at Yahoo! Canada schrieb:
Our AIX admins and DBAs have had some differing opinions on the layout
of our systems and the use of paging space, so I thought I would post
something here to see the opinions of the experts here.

Here is the system details:
===================

- P5 system with 4 processors
- AIX 5.3
- 8 GB of memory
- eight internal SCSI disks set up as two VGs (rootvg X 2, datavg X 6)
- LV mirroring to different disks will be done

The system will host four Informix IDS 11 instances:
- Inventory and Production (OLTP)
- Accounting (OLTP)
- Information Warehouse (mainly read-only lookups and reporting)
- Testing Environment (rarely used OLTP for training new employees)

It has always been suggested to us to isolate the O/S on rootvg and
install our RDBMS and other packages on a separate VG, so we mirror
rootvg between two disks, and use the remaining disks for our data
(datavg).

We are also advised to make paging space equal to the amount of memory
we have, if we have more than 4GB of memory. (This amount is contested
even within IBM!)

Here is the scenario:
===============
We want to organize the installation of the LVs in such a way as to
maximize performance for the daily operations of the production and
accounting instances. Our AIX admin indicates that we should spread
our LVs across the disks to make as many spindles available and avoid
having a hard disk become a bottleneck due to excessive activity. On
the other hand, others are saying that we should avoid putting things
like a paging space on those same disks, so that if the system starts
paging, it will not interfere with the RDBMS activities.

So the thought is to do the following:

hdisk0 <----> hdisk4 = rootvg mirrored (hdisk4 on other controller)
hdisk1,2,3 <----> hdisk5,6,7 = datavg mirrored (5,6,7 on other
controller)

Spread the two main OLTP instances and the IW instance over as many
disks as possible, put the test database in an out of the way area
(perhaps using up the extra space on the rootvg left from putting a
small O/S footprint on a 72GB drive), and spread the paging space over
all disks evenly.


The admins then began a debate about device speeds...Traditionally,
memory was the fastest, disk and tape access were the slowest part of
your system. So these two questions were raised:

1) Is it better to have one 8GB paging space on rootvg, two X 4GB, or
even it out on all disks and have four X 2GB as we were planning?
(Assuming that the paging spaces are mirrored)

2) Should we isolate the two main OLTPs from each other, putting the
production one on two disks (hdisk1, hdisk2) and the accounting on a
separate disk (hdisk3) to prevent them from fighting for contention of
the disks?

Or are we simply getting too picky in our concerns about data
placement on the disks?

I would create 2 G paging space as hd6 in rootvg + 6 paging00 through paging02 on the datavg with 2G each placing it at the inner edge. This will also prevent the lv's of the database using the slowest parts of the disks ;-)

Don't really concern the I/O into the paging space. If your system starts paging relevant amounts then consider buying RAM or even more RAM is the very best option.
If your paging space usage is below 0.5Gbyte this paging will be very static and therefore not produce significant I/O on the disks. It will simply make a larger address space available.

The (very) old rule of thumb having 1.5 to 2 times amount of RAM as paging space is obsolete.

--
Uli
.

Relevant Pages

  • Paging Space Placement Question
    ... Our AIX admins and DBAs have had some differing opinions on the layout ... LV mirroring to different disks will be done ... Accounting (OLTP) ... We are also advised to make paging space equal to the amount of memory ...
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