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

[Oliver Ransom]

On 19/09/2009, at 6:28 PM, Fabian Arrotin wrote:

> 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 ?

Hi,

I did not say I could not find any documentation covering LVM admin  
for CentOS. I said I could not find any documentation explaining  
exactly how, in the context of my message, the "decision" was made.

I used the default configuration examples from the LVM HOWTO which  
results in a linear arrangement. My question would not have come up if  
I had set things up in a striped arrangement.

The link below says "The physical storage is concatenated".
http://www.centos.org/docs/5/html/5.2/Cluster_Logical_Volume_Manager/linear_volumes.html

That doesn't really answer the question from my example though, if it  
was concatenated and I added md3 before md9, shouldn't it have been  
writing to md3 first? That's what I would have expected.

Oliver


> -- 
> --
> Fabian Arrotin
> idea=`grep -i clue /dev/brain`
> test -z "$idea" && echo "sorry, init 6 in progress" || sh ./answer.sh
>
>
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos