LVM backed drives should default to cache='none'
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
QEMU |
Invalid
|
Undecided
|
Unassigned | ||
virt-manager |
Won't Fix
|
Medium
|
|||
virt-manager (Ubuntu) |
Triaged
|
Wishlist
|
Unassigned |
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://<email address hidden>
http://
http://<email address hidden>
http://
Changed in virt-manager: | |
importance: | Unknown → Medium |
status: | Unknown → Won't Fix |
The default is cache=writethrough in the interest of data integrity.
I don't think we want to differ from what upstream KVM provides, on
this point.
Note that the manpage says:
Some block drivers perform badly with cache=writethrough, most
cache= writeback should be used with qcow2.
notably, qcow2. If performance is more important than correctness,
If you believe that this default should be changed, please have that
discussion on the upstream kvm and qemu mailing lists. I believe that
upstream has discussed this and has chosen data integrity over
performance as the default.
Thanks,
:-Dustin