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

[Ross Walker]

On Sep 19, 2009, at 2:15 AM, Oliver Ransom <oliver at 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