Mirroring + Striping Question

[Shaixpeare]

Hi all,
I have a rather large DB2 database logical volume that I want to stripe over 3 72 Gig SCSI disks on 3 different SCSI controllers. I also want to mirror this logical volume on 3 other 72 Gig disks spread over these same controllers. How do I insure that both copies of a stripe or partition aren't on disks attached to the same controller? Do I have to define a partition map or what? Thanks for any input!

 

[Yegolev]

I would set my vg up with the first 3 disks and get the stripes set up the way you want them. Next extendvg the 3 other disks. Somewhere in `smit lvm` under the volume group section there is a mirroring section. If you use the exact mapping option and the "PP copies on separate disks" option you will have a duplicate of your PP distribution on the second set of three disks. IBM Certified -- AIX 4.3 Administration
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[Shaixpeare]

Thanks for the response! If I do this though, my copies will be on different disks but won't both copies be on disks hooked up to the same controller? I'm trying to protect against a controller failure. Is there a simple way to insure the stripes on disk1 are always mirrored to disk 5 or disk6 (which are attached to a different controller) in the following configuration?

scsil scsi2 scsi3 <---SCSI controllers
disk1 ---- disk2 ---- disk3 <---Vertical striping
disk4 ---- disk5 ---- disk6 <---Mirrored copy

Thanks!

 

[Yegolev]

Sure, it works the same way. You just make your vg so it only contains hdisk1, hdisk2 and hdisk3. Three disks, three adapters. Set up your lv-s and fs-es. When you have something you like, `extendvg -f vgname hdisk4 hdisk5 hdisk6`. Then mirror. Be sure to use the exact PP mapping and the second set will have, um, an exact PP map.

Now, if you are asking how to put three disks on three different adapters, that is an entirely different question. I don't know right off how to do it, but I'd suggest reading about `mkdev`. IBM Certified -- AIX 4.3 Obfuscation

 

[Shaixpeare]

Thanks again. I may not be clearly stating what I'm trying to achieve here though. I've got six disks and 3 controllers, 2 disks attached to each controller. I'm going to create one logical volume that is striped across 3 disks, 1 from each controller. Then I want to mirror those 3 to a second set. I can do this no problem and am protected in case of a disk failure. The potential problem occurs though when mirrored copies of my stripes or partitions are on disks attached to the same controller. Then if I lose the controller, I lose the entire logical volume. I've done a lot of mirroring of disks in the past, but this is my first attempt at combining striping and mirroring...

 

[Yegolev]

I think I get it. You will have, say, hdisk0 and hdisk3 on scsi0, but you want the PPs on hdisk0 mirrored to either scsi1/hdisk4 or scsi2/hdisk5? I'll have to think about this for a few mintutes. IBM Certified -- AIX 4.3 Obfuscation

 

[Yegolev]

I think I have devised a half-ass method involving undocumented commands. It will work (I think) but you'll really want a seventh scratch disk of similar size. The problem is that I have found commands to migrate PPs rather than merely copy them, but I can do what you want by playing a shell game. IBM Certified -- AIX 4.3 Obfuscation

 

[Shaixpeare]

Thanks for trying but we really need the mirroring. I'm beginning to think that a partition map may be the only way to do this. Any other ideas out there? Thanks

 

[Yegolev]

It should be mirrored after I am done, but who knows? Tell you what, I'll see if we have something similar (probably not) and I'll try to simulate your issue. The summary is this:

Stripe and mirror as I outlined earlier.
Use undocumented migration techniques to migrate contents of one disk to scratch disk. Migrate another disk to new empty disk. Repeat. Migrate scratch disk to empty production disk. Remove empty scratch disk.

This should scramble the adapter-disk combos. Without testing I cannot guarantee the mirror will still work. The thing is, I think most people who are trying to do something like this use RAID, or even SSA JBOD. We only mirror, and then we only mirror local disks since just about everything is kept on EMC frames, else on SSA drawers.

I'll play with it in my free time and get back to you. IBM Certified -- AIX 4.3 Obfuscation