[CentOS] How does LVM decide which Physical Volume to write to?

Hi everyone.

This isn't specifically a CentOS question, since it could apply for any distro but I hope someone can answer it anyway.

I took the following steps but was puzzled by the outcome of the test at the end:

1. Create a RAID1 array called md3 with two 750GB drives 2. Create a RAID1 array called md9 with two 500GB drives 3. Initialise md3 then md9 as physical volumes (pvcreate) 4. Create a new volume group called "3ware" with md3 (helps me remember what controller the disks are on) 5. Use vgextend and add md9 to the 3ware volume group. 6. Add a logical volume filling the volume group then create a ext3 filesystem on the entire volume.

Now I started moving a lot of data onto the volume and iostat said all the data was being written to md9. Why that array? How does it decide which physical volume to write to?

I could not find any documentation or information online about how exactly this works.

Thanks!

Oliver