[Xen-devel] qemu-xen-traditional: NOCACHE or CACHE_WB to open disk images for IDE

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

Hi list,

Recently I was debugging L2 guest slow booting issue in nested virtualization 
environment (both L0 and L1 hypervisors are all Xen).
To boot a L2 Linux guest (RHEL6u2), it will need to wait more than 3 minutes 
after grub loaded, I did some profile, and see guest is doing disk operations 
by int13 BIOS procedure.

Even not consider the nested case, I saw there is a bug reporting normal VM 
boots slower than before (actually both qcow and disk image), see:
http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1821
Therefore I think the boot delay is just much lengthened in L2 guest.

I root caused this issue to a change in qemu, and I saw there is a lot of 
discussions on this topic. I didn't see the final decision but later the patch 
was checked in. Could anybody helps to revisit this commit and explain the 
final decision?
http://lists.xen.org/archives/html/xen-devel/2012-03/msg02072.html

Thanks,
Dongxiao

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