mirroring and alternate links

[Michael Murphy_2]

We have a n-class with two fiber channel cards and two fc cables. We use mirroring as well as alternate links. When creating the mirror is it better to use the second fc path as the mirror (for performance) or is it OK to have the primary and mirror go over the same fc card and fc cable? (Does LVM know to use the alternate path when available to increase performance?) Thanks...

[James A. Donovan]

If you do not have a third party product like EMC PowerPath or Veritas DMP, then I would use the second fc path for your mirror disk. LVM uses alternate links strictly for fail-over situations. It does not know how to dynamically load balance data transfers.

[Jeff Schussele]

Hi Michael,

ABSOLUTELY NO DOUBT - use the second path. You'll eliminate three more SPOFs - the HBA, the switch/array I/F & the fibre(s). But I assume you mean a second channel to *another* LUN - correct?

The alt link is used - somewhat - for read performance i.e. the LUN with the smallext queue will get the next request. Of course all writes are doubled up.

Rgds,
Jeff

[Mark Grant]

For performance reasons, you want the disks in your volume group spread across the two fibre cards. You also want the mirrors spread across them too. On the other hand, if you only have two disks in your volume group, you'll have to have one disk on one card and the mirror on the other in order for the mirroring to make any sense.

[Sridhar Bhaskarla]

Hi,

LVM knows to use alternate path only when the primary path fails. So, unless you specify the alternate path explicitly for the mirror, it will continue to use the primary path. The following would be my configuration. Say c2t0d0 and c2t0d1 is mirror pair and the corresponding alternate paths are c4t0d0 and c4t0d1, then I would make sure my lvmtab reflects the following order.

c2t0d0
c4t0d1
c4t0d0
c2t0d1

This way both performance and the availability are covered. If you already added the disks, you can achieve the above configuration by successive vgreduce and vgextend commands.

-Sri

[Jeff Schussele]

Oops - sorry that should have read.

The *mirror* pair is used - somewhat - for read performance i.e. the LUN/disk with the smallest queue will get the next request. Of course all writes are doubled up.

The alt link is only used at failure.
And we certainly also do "poor man's load balancing" by alternating link order through the LUNs in the VG. It's not as effective as a product like PowerPath, but under heavy load - you can certainly see the benefit.

Cheers,
Jeff

[Geoff Wild]

One more thing - it would be better to mirror at the Hardware level (EMC, WHY)...

Especially for large LUNS....

Then, if you don't have a tool like Powerpath, you can manually load balance your disks with alternate links...

IE - put 50% through one HBA and 50% through the other.

Rgds...Geoff