[Qemu-devel] [Bug 524447] Re: virsh save is very slow

[Anthony Liguori]

This actually turns out to be related to dd's default block size.  By
default, dd uses a block size of 512.  The effect of this is that qemu
fills the pipe buffer very quickly because dd just is submitting very
small requests (that will require a RMW).

If you set an explict block size with dd (via bs=1M), you'll notice a
significant improvement in throughput.

So I think this turns out to be a libvirt issue, not a qemu issue.

** Changed in: qemu
       Status: Confirmed => Invalid

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virsh save is very slow
https://bugs.launchpad.net/bugs/524447
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Status in QEMU: Invalid
Status in “qemu-kvm” package in Ubuntu: Confirmed

Bug description:
As reported here: 
http://www.redhat.com/archives/libvir-list/2009-December/msg00203.html

"virsh save" is very slow - it writes the image at around 1MB/sec on my test 
system.

(I think I saw a bug report for this issue on Fedora's bugzilla, but I can't 
find it now...)

Confirmed under Karmic.