On Sep 19, 2009, at 2:15 AM, Oliver Ransom oliver@ransom.com.au wrote:
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:
- Create a RAID1 array called md3 with two 750GB drives
- Create a RAID1 array called md9 with two 500GB drives
- Initialise md3 then md9 as physical volumes (pvcreate)
- 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.
I don't know how it picks a PV, maybe flips a coin, but it isn't suppose to be a concern to the admin.
One can explicitly assign a PV during LV creation by specifying the PV behind the VG name. If you want to stripe the PVs look at the -i and - I options to lvcreate. If you have a lot of PVs you can specify which PVs to stripe across by listing them after the VG name.
-Ross