FC10 RAID 1 and Channel Seperation

[Shawn_4]

Gang:

Newbie here - be gentle !

Setup:

I have one L2000 class box, two fibre channel controllers connected to one FC10 10x18GB disk array.

Goal:

To have one huge lvol mirrored within a single volume group on this array (RAID 1)...

Question:

Do I have to do anything special to acheive full channel seperation in this configuration ?

If I do an ioscan - I can see two controllers and twenty drives ( viewed through each controller).

If I view through SAM - it appears as 10 drives and one controller.

Does LVM and the Tachyon Fibre Channel bundle automatically provide for channel seperation and load balancing and therefore all I need to do is create a VG and mirrored LV though SAM or do I have to create two PVG's with five drives each ?

Hope this makes sense !

Shawn.

[Andreas Voss]

Hi,

i would do a vgdisplay -v to see if the vg has alternate links in it.
If not you can add this with the device files listed from ioscan and doing vgextend

Regards

[Shawn_4]

Andreas:

I haven't created any VG's yet - I just want to get this straight before I start..

Let's remove SAM from the picture for a moment and talk about pvcreate ...

In my configuration - disk0 on the disk array has two sets of device files ...

/dev/dsk/c4t0d0 and /dev/rdsk/c4t0d0

and

/dev/dsk/c5t0d0 and /dev/rdsk/c5t0d0

same disk - two controllers ?

If I was to pvcreate this drive - would I just flip a coin and pick heads or tails in terms of which controller to use ?

Or - more importantly - am I missing some important piece of understanding or software that provides for a higher level of device abstraction - one that views these drives as single devices ?

Shawn.

[Andreas Voss]

Hi,

i'm not familar with the FC10 but i would do:
pvcreate /dev/rdsk/c4t0d0
And then look with:
pvdisplay /dev/dsk/c4t0d0
pvdisplay /dev/dsk/c5t0d0
if the output is the same.
When it's the same, SAM should recognize the alternate link at vg creation time.


Regards.

[Shawn_4]

Anreas:

OK - I tried what you suggested and successfully created the PV on /dev/rdsk/c4t0d0 however - when I ran pvdisplay - it sqwaked about it not belonging to a VG and wouldn't display anything...

More importantly - your dialog has led me to a better understanding of alternate links concept (the higher level of abstraction I was looking for).

Re-looking at SAM now - I can see that it does indeed show only ten drives (like I said before) - but has set the "number of paths" to 2.

Having said that - I guess I can now proceed to create my VG and LV mirror safe in the knowledge that LVM recognizes (and accounts for) the alternate links.

Only one question begs - in reading about alternate links - one link is termed the primary and one the secondary - does LVM attempt to load balance across alternate links - or only use the alternate link as a spare.

Seems a huge waste of bandwidth if the latter is true. Is there anyway to force the load balancing issue ?

Shawn.

[Andreas Voss]

Hi,

for your balancing question look at this thread:
http://forums.itrc.hp.com/cm/QuestionAnswer/1,1150,0x54d7f841489fd4118fef0090279cd0f9,00.html

i put you a script for viewing LVM information at attachment.

Regards

[Andreas Voss]

Hi,

for your balancing question look at this thread:
http://forums.itrc.hp.com/cm/QuestionAnswer/1,1150,0x54d7f841489fd4118fef0090279cd0f9,00.html

i put you a script for viewing LVM information at attachment.

Regards

[Steve Massey_1]

Hi

There are many previous thread covering this question - use the SEARCH page and click on Forums.

But a recap alternate links are just that, and will come into play when the primary link goes down.
But if you have many disks in a Volume Group you can use alternate controllers as he primary ie c3t0d1 then c2t0d2 utilising controller c2 and c3 as primary links.

Often used when striping across many disks

Hope this helps

Steve

[Shawn_4]

Steve:

Yup - I'm aware that there's a search function on "da-board" here - but it helps if one knows what one is searching for ! - The concept of "Alternate Links" was foreign to me...

Steve/Andreas:

Thanks in part to the both of you - I now have one way fast bitch'n FC10 disk array - locked and loaded - running software RAID 0+1 with extent-based mirror stripes.

Only one last question before you earn your 10 points ...

From reading the post - it would seem that the ordering of alternate links is controlled through a JASFF called /etc/lvmpvg (Just A Simple Flat File ;) )

If I wanted to change the alternate links - is it just a matter of editing this file - tweaking the ordering - then saving it back and re-booting. Or should I unmount the file system first - then do the above.

Shawn.

[Andreas Voss · Accepted solution]

Hi,

the order of the devices are controlled by the vg.
When you look at vgdisplay -v /dev/vgXX
you get similar:
--- Physical volumes ---
PV Name /dev/dsk/c8t15d3
PV Name /dev/dsk/c9t15d3 Alternate Link

Now to change the order just:
vgreduce vgXX /dev/dsk/c8t15d3
that switches the /dev/dsk/c9t15d3 to primary (you get a message of this)
and then:
vgextend vgXX /dev/dsk/c8t15d3
will add /dev/dsk/c8t15d3 as the alternate link.

Regards