VxVm or LVM Extent Based Stripping

[Alun Territt_1]

I am currently running Oracle databases on HP-UX 11.0

I would like to stripe some of the disks used for the databases - The question is, how do I choose between 11.0 extent based stripping or to upgrade to 11i and use Veritas volume manager.

I would have to identify key benefits of Veritas in order to justify the upgrade to managment.

Any input would be much appreciated.

Regards

Alun

[Stefan Farrelly]

Theres quite a few things to consider here;

1. Why do you want to stripe ? I guess you are seeking better I/O performance ? The only way you are going to get striping to give you better I/O performance is if you have more channels to disks. ie. If you have several disks available but only over 1 or 2 controllers (SCSI or fiber) then youre only going to increase performance a little. What you really need is disks across something like 4+ channels which you can stripe over, this will give a big I/O boost.

2. What kind of disks are you using ? SCSI, fiber ? If Fibre then the performance is already about as fast as it will go, striping isnt really going to help too much across multiple fibre channels as the disks just wont be able to keep up with the extra throughput from striping (fibre channel is already very quick and has large throughput).

3. If you still want to stripe then you have 3 ways to do it;
a. lvcreate -i -I You can do this already under LVM - no extra software needed, all you have to do is recreate your logical volumes.
b. lvextend 1 extent at a time over all available disks/controllers. This is slow to create and needs careful maintenance, better to use the lvcreate -i -I above.
c. VxVM. This is a costly piece of software - especially if youre only going to use if for striping. Why pay all that money when you can stripe already under lvm. VxVM is really for advanced file system options and tuning. I know hardly anyone who is bothering to use it.

[Alun Territt_1]

Stefan

Thanks for your repsonse.

To clarify, I want to ensure that I have even distribution of I/O over all my disks. At present, one of the disks is heavily hit and I want to even out the distribution rather than having to identify the hot table and move it to another disk (only to then find another table is becoming heavily hit). Also, I would like to eliminate the issue of growth on disks - at the moment a few of my disks are reaching capacity whilst other have lots of spare capacity.

I was under the impression that as Veritas is bundled in as a volume manager utility - HP were looking to phase out LVM - is this not the case.

My question from above, is given the need for striping and mirroring. Should one use Veritas or LVM (extent based stripping).

I take it extent based striping is still the only way in order to stripe and mirror with LVM.

Regards

Alun

[Duncan Edmonstone · Accepted solution]

This is an ineteresting post on LVM vs. VxVM:

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xfb25cbaac6dcd5118ff40090279cd0f9,00.html

HTH

Duncan

[Stefan Farrelly]

Hi Alun,

Yes, striping to ensure an even load over your disks is a good reason to make the effort.

HP Make LVM. Veritas make a replacement for it, VxVM. I believe HP originally intended to phase our LVM and replace it with VxVM but as HP supply LVM free and VxVM costs a lot of money I think HP have decided to stick with LVM as part of the OS, and only if you want extra features such as online filesystem resizing, advanced tuning options etc, do you purchase VxVM.

Seeing as HP support LVM and it comes with striping for free I suggest you use that. If you have the money you can purchase VxVM but you will need to learn all new commands (which replace HP's vg.. and lv.. commands) and if youre only going to use if for striping it seems to be overkill to me.

[Alan Nicholson]

As a variation of Stefan's reply b) above, you could do the following:

An lvcreate -D option was introduced to HP-UX 10.20 with LVM patch PHCO_18563 and to HP-UX 11.0 with PHCO_16851.

In order to make use of the -D option you must create the volume group with physical volume groups, e.g. one PVG with all of the primary disks in it and one with all the mirrors.

The distribution policy of a logical volume may be set with the lvcreate -D option. You must set this option to "y" (yes) and also set the PVG-strict allocation policy (-s g).

With the distribution policy set and the PVG-strict allocation policy, allocation of free extents will occur in a round-robin fashion across the disks in the volume group (creating your stripes). If you allocate your mirror(s) at the same time they will be allocated from a different PVG.

Example:

# vgcreate -g Primary -s 1 /dev/vg01 /dev/dsk/c1t5d0 /dev/dsk/c1t4d0 /dev/dsk/c1t3d0
# vgextend -g Mirror /dev/vg01 /dev/dsk/c0t5d0 /dev/dsk/c0t4d0 /dev/dsk/c0t3d0
# lvcreate -D y -s g -m 1 -L 500 /dev/vg01


Whilst is would be best to stripe over several controllers, we have noticed significant performance improvements using this technique with serveral disks on the same controller.

[Alan Nicholson]

Forgot to say, if you use the strict disribution policy you could run into problems when you have almost allocated all of the available space in the volume group. If all of the available space is on one physical disk, you will not be able to create a logical volume because the strict allocation would not allow consecutive extents on the same disk.

Hope that makes some sense. :)