Oliver Ransom 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:

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.

What ? no documentation covering LVM admin on/for CentOS ? hmm, is http://www.centos.org/docs/5/html/5.2/Cluster_Logical_Volume_Manager/ not reachable from your side ? How have you configured you LV ? linear or stripped ?

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:

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.

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