alternate link to vg

[siva baskaran]

dears,

what is the use of alternate link to vg then how to do it

thanks
siva

[Patrick Wallek]

You can have alternate links from your machine to your physical volumes. You do this by doing multiple vgextends. HP-UX will automatically detect when a disk device is an alternate link to a device already in the VG.

Having multiple paths is useful because it reduces your exposure for downtime. If you have a single path and you lose a controller, cable, fibre channel switch or any single component, then your data is no logner accessable. If you have multiple paths, then any single component failure (other than a catastrophic disk array failure) should not result in data unavailability.

If set up correctly, multiple paths to a disk / disk array can also increase throughput as you have requests flowing down multiple data channels.

[James R. Ferguson]

Hi Siva:

LVM altername links (otherwise known as 'pvlinks') provide high-availablity to a physical device. That is, should one path to a disk fail, another link (of up to eight (8), or seven (7) alternate and one primary) takes over to provide access.

LVM does not concurrently balance I/O among the primary and alternate paths, although when one has a disk array with multiple physical volumes and two or more paths (pvlinks) one does gain throughput by configuring one primary path to service half the disks from one disk controller and configuring another primary path to service the other half of the disks from a separate controller. With each controller offering an alternate link, there is no single point of failure since each alternate link can take over should its primary fail.

To configure an alternate link, simply 'vgextend' the volume group using a device path. To promote an alternate link to a primary (which is to say to switch an alternate and a primary link), 'vgreduce' the primary link and then 'vgextend' the link back in.

Regards!

...JRF...

[David Bellamy]

Also siva you must do the vgextend from the command line because sam does not recognize alternate links

[Senthil Kumar .A_1]

Hi Siva,

There is nothing called "Alternate link" to VG per say. I think you meant Alternate link to PV. This feature in LVM is termed "PV Link". This operates in "Active/Passive" mode Meaning, here the alternate link is used only when the primary fails. It can't be used for Load balancing scenarios.

We do not have a special command in LVM to operate on the PV links. Its only that, from the storage commands like "xpinfo" ot "inqraid" you need to find what H/W paths are alternate to each other. After compiling this list, choose one of the device file for each disk and "pvcreate" on them. Then create VG or add them to existing VG. After doing so, the sceond paths you add them using "vgextend" without performing "pvcreate" ,because the PVRA's and VGRA's are already created. Once done, you will find the output of your "vgdisplay -v" as below...

Truncated the output for better reading...
.
.
.
PV Name /dev/dsk/c6t2d2
PV Name /dev/dsk/c4t2d2 Alternate Link
PV Status available
Total PE 1735
Free PE 0
Autoswitch On
.
.
.

Regards,
Senthil Kumar .A

[Sivakumar TS · Accepted solution]

Hi Sivabaskaran,

Assume that you have a storage Box with 2 controller,

and the server too has 2 HBAs..

when this BOTH are connected together,

the LUNs/Disks in the Storage will be visible to the OS thru both the HBAs and each disk will have 2 instancess,

in that one will be the promary link and other can be the alternate link to the same PV, which can be used to access the disk in case failure of the primary link.

Regards,

Siva.

[siva baskaran]

thanks to all. but can u give some example

like

let us take c6t4d0, c7t4d0 then name of vg is vg01,

please make disk as alternate path to vg01

give some example
thanks
siva

[Torsten.]

If disk device c6t4d0 is already in your VG, you have only to extend the VG with c7t4d0 (the alternate path).

# vgextend -n vg01 /dev/dsk/c7t4d0

now do a

# vgdisplay -v vg01

and you will see the alternate disk.

[Ganesan R]

Hi Siva,

As per your example, if both c6t4d0 and c7t4d0 is pointing to one LUN via two diff paths, consider one as primary path.

lets assume you are going to create VG first time and c6t4d0 is a primary path and c7t4d0 is a alternate path.

# pvcreate /dev/rdsk/c6t4d0
# vgcreate /dev/vgname /dev/dsk/c6t4d0 (assuming u already created the vgdir and group file)
# vgextend /dev/vgname /dev/dsk/c7t4d0
# vgdisplay /dev/vgname -> it should show c7t4d0 as alternate link for this volume group.

As our gurus told already from the os point of view these alternate links are mainly for high availability.

[siva baskaran]

thanks to all

siva