MIRRORING Un-equal LUNs

[Jim Walls]

We need to move a large amount of data from one SAN onto another without interrupting the on-line application. I propose to do this by setting up a mirror accross the two SANs and then, once the mirror is synchronised, reducing the mirror by dropping the LUNs on the "old" SAN.

Unfortunately, the new SAN allocates LUNs in 50Gb chunks, wheras the existing SAN allocates 10Gb LUNS.

The question is, does this matter?

[Patrick Wallek · Accepted solution]

That depends. How is the VG configured? Was it setup originally to handle larger sized volumes?

2 parameters matter in the VG. "MAX PE PER PV" and "PE SIZE".

Do a 'vgdisplay' on the VG. Find the values for those 2 settings. Multiply them together. If the result is smaller than 50GB, then this won't work and you will have to find another way.

[A. Clay Stephenson]

I'll assume that you are talking about LVM mirroring. The answer is that "yes, it probably does". Mirroring occurs at the LVOL level so in that sense the size of the underlying Physical Volume (closely related to a LUN) doesn't matter; however, when a Volume Group is first created the extent size and the maximum number of extents is typically sized to accomodate the largest PV and/or the largest anticipated LVOL. It is very likely that in your case 40GiB of disk space will go unused on each new LUN. Whether or not that is significant it entirely your call.

Do an lvdisplay /dev/vgxx and note the Max PE per PV and PE Size (Mbytes) values. If the product of those two values is less than 50GiB then the extent that the value is less than 50GiB is how much space that will actually go unused per PV (LUN).

Now, if your are running VxVM things are much more flexible.

[Jim Walls]

Many thanks for the confirmation.

Max PE per PV = 65535
PE Size = 64 Mb

Whoever originally set up the VG thought well ahead!

[Jim Walls]

Question answered.

[A. Clay Stephenson]

The Force was with you.