[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
I can confirm Fabian's tip about the cache=writeback tip. I've one more tip for you: if you are using image files for virtual disks, then using ext3 as the filesystem for the host OS will greatly improve the disk I/O of your guest operating systems. With a Lucid+ext4+KVM+WindowsXP guest combination I've measured a write throughput of 1-3 MB/s during a Windows XP installation. Awfully slow. Regardless of the image format (qcow2 or raw) or the virtual I/O controller (IDE or virtio). However using the same parameters with an ext3 host filesystem, I've measured 20-30 MB/s with raw images and IDE controller (note that this was still during a standard Windows XP installation, not in a disk performance test). I've not yet tested qcow2 images and virtio, but I'm pretty certain that they will show similiar performance boost. So either use cache=writeback, or switch back to ext3. Or if you're familiar with ext4 tweaks, you might already know which option you've to disable to get back the ext3-like disk performance of qemu. P.S.: one might argue whether this is a libvirt, a qemu or ... bug. Qemu will most probably not change it's default cache policy, thus I see the best chance for fixing this problem in libvirt. It already handles tons of special cases where this or that option has to be applied if this or that environment is present. I guess libvirt could (silently) use cache=writeback for guests that have their booting hard disk in an image file that is on an ext4 filesystem. P.S.2: virt-manager's (a GUI to libvirt) latest version (0.8.5-1) introduces the option to manually specify the cache mode for a disk in a guest. However the current version honors libvirt's cache mode for disks. Thus if you manually edit libvirt's guest definition (eg. using virsh's edit command) and specify a cache mode for a disk, virt-manager will keep it intact throughout further guest modifications. The virt-manager change for this is here: http://hg.fedorahosted.org/hg/virt-manager/rev/0241648684d1 The virt-manager 0.8.5 changelog is here: http://hg.fedorahosted.org/hg/virt-manager/file/0b54eb67ce31/ChangeLog -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
** Tags added: cache ext3 ext4 libvirt qemu writeback ** Tags removed: amd64 apport-bug -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
I've tested this with Karmic+KVM+ext4 and the problem is present as well. Using Karmic+KVM+ext3 resulted in the same performance boost as on Lucid. I'd be nice if others could comment on whether this problem is present with Lucid/Karmic+Xen+ext4 too. I guess it should be, because the problem seems to be between the default cache=writethrough policy and an ext4 host OS filesystem. And changing either of them solves the issue. Btw. I was using in Lucid qemu-kvm-0.12.3 and in Karmic qemu-kvm-0.11.0. Probably this is not important in this bug. -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
I forgot to add: the cache policy is a QEMU thing and since Xen uses QEMU too, it should be affected as well. -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
With the cache=writeback in a Lucid+KVM+libvirt+WindowXP guest setup I've experienced near host-level I/O performance in the guest. Thus this makes use of a virtio driver unnecessary. Btw. for casual libvirt users ... adding the cache=writeback option means: 1. stop the VM (in case it's running) 2. execute virsh edit guestdomain on the host OS (where guestdomain should be the name of your guest domain) 3. edit the domain XML by adding a driver cache='writeback'/ element under the disk ... device='disk'.../disk element -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.
If you want to read more about QEMU disk cache internals, I've found a short presentation on the topic here: The KVM/qemu storage stack (by Christoph Hellwig) www.linuxfoundation.jp/jp_uploads/JLS2009/jls09_hellwig.pdf -- Slow disk IO for all guests with all disc modes and types. https://bugs.launchpad.net/bugs/550409 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to libvirt in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 404394] Re: kvm-84: virtio on qcow2 broken (regression)
I revoke my previous comments. Meanwhile it turned out that the server I was testing with had HW problems (namely it couldn't handle 6x2GB RAM despite of the motherboard docs claiming it would). So my error reports are unreliable ... they might have been caused by HW issues. -- kvm-84: virtio on qcow2 broken (regression) https://bugs.launchpad.net/bugs/404394 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to kvm in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 404394] Re: kvm-84: virtio on qcow2 broken (regression)
I hope I'm not getting lynched :-), but I'd like to reopen this bug as not fixed. Previously you wrote that Karmic is not affected, however I've experienced filesystem corruption inside qcow2 images. It's a bit hard to reproduce since I had it in Windows 2003 guests, but I'll try to create a test case with an Ubuntu (eg. Karmic) guest too. The host OS is Ubuntu 9.10 Karmic and kvm is version 84+dfsg-0ubuntu16+0.11.0+0ubuntu6.3. The corruption occurs only during high I/O (as described by others before). It's irrelevant whether the high I/O is in the host OS or in the guest. I'll post again once I've an easily reproduceable test case, preferably in a linux guest OS. -- kvm-84: virtio on qcow2 broken (regression) https://bugs.launchpad.net/bugs/404394 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to kvm in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 404394] Re: kvm-84: virtio on qcow2 broken (regression)
Btw: I do not use virtio in any of my guests, only emulated IDE controllers ... which is most probably a big difference compared to the original bug report. -- kvm-84: virtio on qcow2 broken (regression) https://bugs.launchpad.net/bugs/404394 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to kvm in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
[Bug 404394] Re: kvm-84: virtio on qcow2 broken (regression)
I'm also hit by bug#448694 which was the reason for using qcow2 images in the first place. Now I'm pretty much left in the cold ... cannot use RAW or LVM ... and neither qcow2. What am I supposed to store my guest filesystems in then? :-( -- kvm-84: virtio on qcow2 broken (regression) https://bugs.launchpad.net/bugs/404394 You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to kvm in ubuntu. -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs
