4 ms·

I have two iSCSI Targets. I want to pull a LUN from each, and mirror on the KVM Host system, exposing the mirrored disks for my guests. Compared to using ZFS o

by ssmoot 15y ago

I have two iSCSI Targets. I want to pull a LUN from each, and mirror on the KVM Host system, exposing the mirrored disks for my guests. Compared to using ZFS on OpenSolaris with Xen 3.1, this process is incredibly cumbersome and unreliable. Plus I just don't get the need for libvirt. It seems an incredibly complex and useless abstraction over a simply documented configuration file for Guests, and a tool to start/stop/add/remove them. I'm sure a big part of that may just be that I'm not familiar enough with KVM. Still, it's 2011. Tying all your data to a single host with limited redundancy and depending on live-migration seems like a fundamentally flawed approach to me with a lot of needless complexity.

rednaught 15y ago
Libvirt is not needed to use KVM but the purpose has been to provide stability (and an abstraction as you mentioned) over the changes QEMU has had in various releases. Update: Here's a blog related to your very question that was written just a few days ago: http://berrange.com/posts/2011/06/07/what-benefits-does-libvirt-offer-to-developers-targetting-qemukvm/ http://berrange.com/posts/2011/06/07/what-benefits-does-libv...
- lsc 15y ago
  now, the version of KVM I'm using is really, really old, but as far as I can tell, KVM, by itself, does not do any locking. It's pretty easy to start a single guest twice when using kvm by itself, which will irreparably corrupt your instance. So yeah, I'd strongly recommend that you use libvirt (or some other wrapper that handles things like locking the block devices) if you use KVM. With xen, on the other hand, it handles that level locking for you out of box, so personally I see no reason to use libvirt. The libvirt devs seem pretty focused on KVM anyhow; Xen support, at least in the past, was pretty poor, so personally, I use the native xen tools for xen. (I'm not saying this is a reason to use xen instead of KVM; I'm just saying that if you do use KVM, you should also use libvirt.)
  - ciupicri 15y ago
    Regarding KVM locking, this is going to be fixed in the next version of Fedora - Fedora 16 [1]. [1] http://fedoraproject.org/wiki/Features/VirtLockManager http://fedoraproject.org/wiki/Features/VirtLockManager
wazoox 15y ago
> I have two iSCSI Targets. I want to pull a LUN from each, and mirror on the KVM Host system, exposing the mirrored disks for my guests. Compared to using ZFS on OpenSolaris with Xen 3.1, this process is incredibly cumbersome and unreliable. I don't see the relation with KVM. There is nothing special in the way it uses open-iscsi or mdadm to access iSCSI targets and set up mirrors. Maybe you're unfamiliar with these Linux-specific tools and mistakenly attribute to KVM your difficulties which are really Linux problems. > Plus I just don't get the need for libvirt. I don't use libvirt; I wrote a couple of scripts that allow me to do what I need with KVM. libvirt is useful only if you need to manage a whole lot of VMs.
- ssmoot 15y ago
  No, I'm aware they're Linux problems. It just seems the mind-share is on KVM these days so I generally consider Linux Host Virtualization == KVM. You're absolutely correct though. It's not really KVM's fault. It's the Linux iscsi, software RAID, and volume management capabilities that are so lacking. It just happens that that makes using KVM much less appealing. I want: 1. Reliable Snapshots and Replication 2. Simple Volume Management 3. Human readable device names 4. Consistency of volumes/devices between reboots 5. An "uncorruptable" FS backing the guests 6. Reliable Virtualization OpenSolaris is the only platform I know addresses these concerns. Linux doesn't come close despite it feeling like it's virtualization options are more mature. FreeBSD may be a good fit with VirtualBox, but VBox on OpenSolaris and OpenIndiana was unstable for me at least. I'd like to give FreeBSD a try sometime. On libvirt, that's about what I expected. I guess I just wanted to complain about it in general. ;-) sudo xm new /my/config/file sudo xm start my_guest sudo xm shutdown my_guest sudo xm delete my_guest What's going through my head: "I need a tool that I haven't seen a decent one page explanation on to wrap that why?" :-)

by ssmoot 15y ago

rednaught 15y ago

Libvirt is not needed to use KVM but the purpose has been to provide stability (and an abstraction as you mentioned) over the changes QEMU has had in various releases. Update: Here's a blog related to your very question that was written just a few days ago: http://berrange.com/posts/2011/06/07/what-benefits-does-libvirt-offer-to-developers-targetting-qemukvm/ http://berrange.com/posts/2011/06/07/what-benefits-does-libv...

lsc 15y ago
now, the version of KVM I'm using is really, really old, but as far as I can tell, KVM, by itself, does not do any locking. It's pretty easy to start a single guest twice when using kvm by itself, which will irreparably corrupt your instance. So yeah, I'd strongly recommend that you use libvirt (or some other wrapper that handles things like locking the block devices) if you use KVM. With xen, on the other hand, it handles that level locking for you out of box, so personally I see no reason to use libvirt. The libvirt devs seem pretty focused on KVM anyhow; Xen support, at least in the past, was pretty poor, so personally, I use the native xen tools for xen. (I'm not saying this is a reason to use xen instead of KVM; I'm just saying that if you do use KVM, you should also use libvirt.)
- ciupicri 15y ago
  Regarding KVM locking, this is going to be fixed in the next version of Fedora - Fedora 16 [1]. [1] http://fedoraproject.org/wiki/Features/VirtLockManager http://fedoraproject.org/wiki/Features/VirtLockManager

wazoox 15y ago

> I have two iSCSI Targets. I want to pull a LUN from each, and mirror on the KVM Host system, exposing the mirrored disks for my guests. Compared to using ZFS on OpenSolaris with Xen 3.1, this process is incredibly cumbersome and unreliable. I don't see the relation with KVM. There is nothing special in the way it uses open-iscsi or mdadm to access iSCSI targets and set up mirrors. Maybe you're unfamiliar with these Linux-specific tools and mistakenly attribute to KVM your difficulties which are really Linux problems. > Plus I just don't get the need for libvirt. I don't use libvirt; I wrote a couple of scripts that allow me to do what I need with KVM. libvirt is useful only if you need to manage a whole lot of VMs.

ssmoot 15y ago
No, I'm aware they're Linux problems. It just seems the mind-share is on KVM these days so I generally consider Linux Host Virtualization == KVM. You're absolutely correct though. It's not really KVM's fault. It's the Linux iscsi, software RAID, and volume management capabilities that are so lacking. It just happens that that makes using KVM much less appealing. I want: 1. Reliable Snapshots and Replication 2. Simple Volume Management 3. Human readable device names 4. Consistency of volumes/devices between reboots 5. An "uncorruptable" FS backing the guests 6. Reliable Virtualization OpenSolaris is the only platform I know addresses these concerns. Linux doesn't come close despite it feeling like it's virtualization options are more mature. FreeBSD may be a good fit with VirtualBox, but VBox on OpenSolaris and OpenIndiana was unstable for me at least. I'd like to give FreeBSD a try sometime. On libvirt, that's about what I expected. I guess I just wanted to complain about it in general. ;-) sudo xm new /my/config/file sudo xm start my_guest sudo xm shutdown my_guest sudo xm delete my_guest What's going through my head: "I need a tool that I haven't seen a decent one page explanation on to wrap that why?" :-)