KVM, NFS, iSCSI and storage pools

Adon Metcalfe adon.metcalfe at gmail.com
Thu Mar 10 13:48:25 UTC 2011

Proxmox does it quite nicely using LVM ontop of iSCSI targets - the LVM
locking only allows an LV to be accessed by one host at a time, but if its
deactivated on one host it can be reactivated on another


On 10 March 2011 21:02, <jurgen.depicker at let.be> wrote:

> Hi all.
> Presently, 7 VMs (4 windows, 3 ubuntu) are using the same nfs pool.  The
> machine serving that NFS pool (VLET1) has high load as soon as there is some
> continuous disk activity:
> top - 13:25:30 up 23:16,  5 users,  load average: 4.51, 4.41, 3.98
> Tasks: 191 total,   1 running, 189 sleeping,   0 stopped,   1 zombie
> Cpu0  :  3.0%us,  2.6%sy,  0.0%ni, 44.6%id, 49.8%wa,  0.0%hi,  0.0%si,
>  0.0%st
> Cpu1  :  4.2%us,  2.9%sy,  0.0%ni, 54.2%id, 38.1%wa,  0.0%hi,  0.6%si,
>  0.0%st
> Cpu2  :  3.6%us,  2.3%sy,  0.0%ni, 55.4%id, 38.7%wa,  0.0%hi,  0.0%si,
>  0.0%st
> Cpu3  :  2.9%us,  4.2%sy,  0.0%ni, 56.4%id, 36.5%wa,  0.0%hi,  0.0%si,
>  0.0%st
> It's a quad-core Xeon, running SW Raid on sataII disks:
> cat /proc/mdstat
> Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
> [raid4] [raid10]
> md0 : active raid1 sdb1[1] sda1[0]
>       966796224 blocks [2/2] [UU]
> When the VM client's disk activity is low, the load on VLET1 is very
> minimal:
> top - 13:53:30 up 23:44,  5 users,  load average: 0.11, 0.81, 1.83
> Tasks: 191 total,   1 running, 189 sleeping,   0 stopped,   1 zombie
> Cpu0  :  1.0%us,  2.0%sy,  0.0%ni, 92.1%id,  4.6%wa,  0.0%hi,  0.3%si,
>  0.0%st
> Cpu1  :  2.2%us,  5.3%sy,  0.0%ni, 90.7%id,  1.9%wa,  0.0%hi,  0.0%si,
>  0.0%st
> Cpu2  :  4.2%us,  3.6%sy,  0.0%ni, 87.7%id,  4.5%wa,  0.0%hi,  0.0%si,
>  0.0%st
> Cpu3  :  3.2%us,  3.2%sy,  0.0%ni, 93.3%id,  0.3%wa,  0.0%hi,  0.0%si,
>  0.0%st
> What can I do to reduce the load?  Now, sometimes a VM refuses to come up
> (windows blue screen) except when I launch it on the same host (VLET1)
> hosting the nfs share.  After booting the VM, I can live migrate it to
> another VM host, eg VLET2.
> I did spot an error in /var/log/messages at boot failure of the windows VM
> when starting it on VLET2:
> Mar 10 11:06:46 VLET2 libvirtd: 11:06:46.137: warning :
> qemudParsePCIDeviceStrs:1422 : Unexpected exit status '1', qemu probably
> failedbut I'm not sure this is related.
> This is a test config; our company's mail server will soon be running in
> this cluster of KVM hosts (VLET1 and VLET2 will be joined by VLET3 next
> week).  Since the Domino mail server is very disk intensive, I'm a bit
> worried now.  I would rather not run it on local disks, sionce this makes
> live migration impossible.  I'll need to decide where to put the storage
> pool, and using which protocol (NFS or iSCSI).
> Which brings me to my last question: I was wondering whether it would be
> better to use iSCSI instead of NFS?  I started with this, but couldn't get a
> pool defined through virt-manager (it always showed as 100% full, even when
> I created a completely new iSCSI target/lun).  According to e.g.
> http://communities.vmware.com/thread/150775 it doesn't seem to make much
> difference, whether I use iSCSI or NFS, performance-wise.  Anyhow, I don't
> grasp how KVM running on different hosts, connected to the same iSCSI LUN,
> can work?  It corrupts data, having the same LUN mounted on different
> hosts...  I obviously understood something wrong about iSCSI...
> Regards, Jürgen
> --
> ubuntu-server mailing list
> ubuntu-server at lists.ubuntu.com
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Adon Metcalfe
Labyrinth Data Services Pty Ltd
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