Re: [PLUG] help me design my server

[Matt Ayres <matta@tektonic.net>]

John Von Essen wrote:
> Based on all the comments, I think one of your best options is to have 
> a Raid 1 array dedicated to each VM. And also try to balance all the 
> Raid 1 arrays across more then one controller.
>
> If you stick with SATA, you could get two 8-port controllers, that 
> would support 8 individual Raid 1 arrays, 16 drives total, 4 arrays 
> per controller.
>
> It will cost a little more price per Gb wise, but I think it will 
> offer the most performance for the price. With each VM getting its own 
> raid array, it will almost impossible for one VM to tax performance of 
> another.
>
> -John
There is no need to be this static or complex -- simply create RAID (-10 
would be preferred) arrays and use LVM.  I read through this thread and 
it seems no one has suggested using LVM yet.  With LVM you can easily 
combine multiple RAID arrays into a single volume group and then you 
have a fully dynamic storage system and even the ability to create LVM 
snapshots for easy backups.

As others have stated, don't use file-backed storage -- since the files 
are part of the hosts filesystem it _will_ cause double caching as the 
data will be cached inside the VM and also on the host (dom0). Using LVM 
provides equivalent of a raw device and the blocks are passed through 
without any caching/buffering being performed in dom0.

Use a dedicated drive for the VM's swap area, this will (marginally) 
help to protect the VMs that don't swap from those that do. Xen has a 
limited disk I/O QoS system in place that uses a thread per block device 
that is then scheduled with the kernels I/O elevator (CFQ is 
recommended). I wouldn't rely on it for any sort of guarantees.

Regards,
Matt
___________________________________________________________________________
Philadelphia Linux Users Group         --        http://www.phillylinux.org
Announcements - http://lists.phillylinux.org/mailman/listinfo/plug-announce
General Discussion  --   http://lists.phillylinux.org/mailman/listinfo/plug