[Qemu-devel] [Bug 568445] Re: LVM backed drives should default to cache='none'

[Jamin W. Collins]

@Anthony,

I'm aware that I can manipulate the cache settings via libvirt's XML.
That's currently what I've been doing, manually after every VM creation.
However, my point is that qemu clearly recommends that caching not be
used with disks stored on raw volumes.  Additionally, virt-manager does
not provide any means of disabling caching during or after VM creation.
I disagree with your assertion regarding cached IO being faster with
KVM.  All of my tests indicate a multiple fold increase in performance
with caching disabled.

I fail to see how caching provides and more data integrity than no
caching.  Unless I'm mistaken, no caching provides more integrity by
definition.  Now, if no caching also provides a mutli-fold performance
increase (which it does, as qemu's pages even indicate) why so much
resistance to making it the default?

-- 
LVM backed drives should default to cache='none'
https://bugs.launchpad.net/bugs/568445
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Status in QEMU: Invalid
Status in “virt-manager” package in Ubuntu: Triaged

Bug description:
Binary package hint: virt-manager

KVM guests using LVM backed drives appear to experience fairly high iowait 
times on the host system if the guest has even a moderate amount of disk I/O.  
This translates to poor performance for the host and all guests running on the 
host, and appears to be due to caching as KVM defaults to using writethrough 
caching when nothing is specified.  Explicitly disabling KVM's caching appears 
to result in significantly better host and guest performance.

This is recommended in at least a few places:
http://www.mail-archive.com/address@hidden/msg17492.html
http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/48471
http://www.mail-archive.com/address@hidden/msg30425.html
http://virt.kernelnewbies.org/XenVsKVM