CentOS - Sep 2009 - 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

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 ?
-- 
--
Fabian Arrotin
idea=`grep -i clue /dev/brain`
test -z "$idea" && echo "sorry, init 6 in progress"
|| sh ./answer.sh

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

Hi,

On Sat, Sep 19, 2009 at 02:15, Oliver Ransom <oliver at ransom.com.au>
wrote:
> 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?
This does not exactly explain how it picks them at creation, but you
can use the command "lvdisplay -m" or "lvdisplay -m
/dev/vgname/lvname" to show which logical blocks are mapped to which
physical blocks of the underlying pv.

HTH,
Filipe