emc alternate links .. best practice

[someone_4]

Just a quick question about creating vgs using sam vs the command line.

This was created using just the command line:
# vgdisplay -v /dev/apps_vg
--- Physical volumes ---
PV Name /dev/dsk/c6t8d5
PV Status available
Total PE 2157
Free PE 0
Autoswitch On



# vgdisplay -v /dev/apps_vg
on another server this was created using sam.

--- Physical volumes ---
PV Name /dev/dsk/c4t9d0
PV Name /dev/dsk/c6t9d0 Alternate Link
PV Status available
Total PE 2157
Free PE 0
Autoswitch On



same layout.


This is an EMC SYMMETRIX device.

I realize that the alternate links are there in the background.

If i create them using the command line the pvdisplay output does not show Alternate Link.

But if i create them using SAM Alternate Link is in the pvdisplay output.

But is there really a difference? Is there a best practice ?

Thanks
Richard

[A. Clay Stephenson · Accepted solution]

Best practice is to have alternate links; you simply didn't do quite enough from the command line.

vgcreate /dev/vg00 /dev/dsk/c3t0d0 /dev/dsk/c4t0d0

In this case, c3t0d0 and c4t0d0 refer to the same LUN and the order in which they are listed is important. The first one becomes the primary path and the second one becomes the alternate path.

vgextend also does this. By alternating the primary and alternate paths of the PV's which make up a given VG, it is possible to achieve a level of load balancing.

man vgcreate, vgextend for details.

[Sameer_Nirmal]

Hi Richard,

Having latest SAM patches, SAM detection of primary path and alternate path for EMC array should not be a problem. However, SAM does the selection based on the order of the "ioscan -fnC disk".

This *auto* selection need not be preferred one. I would prefer to go with command line option as I would be knowing those paths from array, server IO perspective. I am not SAM fan though.

[someone_4]

Thanks for the quick response Mr Clay.

But I was told that the powermt takes care of the alternate links in the background. And that it does not matter that they "Alternate Link" shows up on the pvdisplay. Is this right?

I would think that the should show up in the pvdisplay. It confirms that the system knows that the ALT link is there.


# powermt display dev=all
Symmetrix ID=000187870452
Logical device ID=0696
state=alive; policy=SymmOpt; priority=0; queued-IOs=0
==============================================================================
---------------- Host --------------- - Stor - -- I/O Path - -- Stats ---
### HW Path I/O Paths Interf. Mode State Q-IOs Errors
==============================================================================
4 0/5/1/0/4/0.100.122.19.0.8.5 c4t8d5 FA 14cB active alive 0 0
6 0/2/1/0/4/0.99.109.19.0.8.5 c6t8d5 FA 3cB active alive 0 0


Thanks
Richard

[Michael Steele_2]

I hate SAM but in this case it certainly appears superior by factoring this command:

vgextend apps_vg /dev/dsk/c6t9d0

[someone_4]

Just ran the commands though sam and looked at the sam viewer.

Sam runs several insf commands
pvcreates

vgcreate to add the primary disks.

/sbin/vgcreate -e 2158 -l 255 -p 16 -s 4 /dev/apps_vg \
/dev/dsk/c4t9d0 /dev/dsk/c4t9d1 /dev/dsk/c4t9d2

As mentioned earlier vgextends to add the alternate links.

/sbin/vgextend /dev/apps_vg /dev/dsk/c6t9d0


Richard

[someone_4]

I am still wondering if the PowerPath takes care of the alternate links in the background without specifying them on the commmand line.


Richard

[someone_4]

Running powerpath with emc

- Richard

[Deoncia Grayson_1]

I'm sure when you run a powermt dispaly dev=all you see the primary and alternate path to each disk but as for as the operating system, it doesn't recognize the alternate path just because power path does. It important that you do a vgextend in order to see your primary and alternate paths. In order words, powerpath knows there are 2 paths but the operating system doesn't.
I heard the same rumor the powerpath sees the alt path and if you lose one path, it will redirect the os to the alt path but nobody yet has fully explained to me how does the operating system knows about this new path. I would do a vgextend, that way I know the os knows about both paths.

[Bill Costigan]

If you are using powerpath, I wouldn't use the alternate links.

Without powerpath it is up to LVM to detect that a path failed and it needs to know what the alternate path is. (ie., alternate paths)

With powerpath LVM will never see a failed link. Powerpath intercepts the I/Os for each of the paths and directs it to the path it prefers. If all paths are up, I think powerpath will use the path that LVM requests.

Although I do not believe the symmetrix has this problem, I've heard that using BOTH powerpath and LVM alternate links together with the clariion can cause performance issues. As LVM tries each link, it causes Powerpath to force the Clariion to tresspass the LUNs between controllers. This causes more overhead and and delays.