One major disadvantage is lack of multipath support.
Multipath is still done outside of qemu and there is no native multipath
support inside of qemu from what I can tell. Another
disadvantage is that qemu iSCSI support is all s/w based. There are
hardware iSCSI initiators that are supported by os-brick today. I think
migrating attaches into qemu itself isn't a good idea and will always be
behind the level of support already provided by the tools that have been
around forever. Also, what kind of support does QEMU have for target
portal discovery? Can it discover all targets via a single portal, and
can you pass in multiple portals to do discovery for the same volume?
This is also related to multipath support. Some storage arrays can't do
discovery on a single portal, they have to have discovery on each interface.
Do you have some actual numbers to prove that host based attaches passed
into libvirt are slower than QEMU direct attaches?
You can't really compare RBD to iSCSI. RBD is a completely different
beast. The kernel rbd driver hasn't been as stable and as fast as the
rbdclient that qemu uses.
Walt
On 06/15/2016 04:59 PM, Preston L. Bannister wrote:
QEMU has the ability to directly connect to iSCSI volumes. Running the
iSCSI connections through the nova-compute host *seems* somewhat
inefficient.
There is a spec/blueprint and implementation that landed in Kilo:
https://specs.openstack.org/openstack/nova-specs/specs/kilo/implemented/qemu-built-in-iscsi-initiator.html
https://blueprints.launchpad.net/nova/+spec/qemu-built-in-iscsi-initiator
From looking at the OpenStack Nova sources ... I am not entirely clear
on when this behavior is invoked (just for Ceph?), and how it might
change in future.
Looking for a general sense where this is headed. (If anyone knows...)
If there is some problem with QEMU and directly attached iSCSI
volumes, that would explain why this is not the default. Or is this
simple inertia?
I have a concrete concern. I work for a company (EMC) that offers
backup products, and we now have backup for instances in OpenStack. To
make this efficient, we need to collect changed-block information from
instances.
1) We could put an intercept in the Linux kernel of the nova-compute
host to track writes at the block layer. This has the merit of working
for containers, and potentially bare-metal instance deployments. But
is not guaranteed for instances, if the iSCSI volumes are directly
attached to QEMU.
2) We could use the QEMU support for incremental backup (first bit
landed in QEMU 2.4). This has the merit of working with any storage,
by only for virtual machines under QEMU.
As our customers are (so far) only asking about virtual machine
backup. I long ago settled on (2) as most promising.
What I cannot clearly determine is where (1) will fail. Will all iSCSI
volumes connected to QEMU instances eventually become directly connected?
Xiao's unanswered query (below) presents another question. Is this a
site-choice? Could I require my customers to configure their OpenStack
clouds to always route iSCSI connections through the nova-compute
host? (I am not a fan of this approach, but I have to ask.)
To answer Xiao's question, can a site configure their cloud to
*always* directly connect iSCSI volumes to QEMU?
On Tue, Feb 16, 2016 at 4:54 AM, Xiao Ma (xima2) <xi...@cisco.com
<mailto:xi...@cisco.com>> wrote:
Hi, All
I want to make the qemu communicate with iscsi target using
libiscsi directly, and I
followed https://review.openstack.org/#/c/135854/ to add
'volume_drivers =
iscsi=nova.virt.libvirt.volume.LibvirtNetVolumeDriver’ in nova.conf
and then restarted nova services and cinder services, but still
the volume configuration of vm is as bellow:
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source
dev='/dev/disk/by-path/ip-10.75.195.205:3260-iscsi-iqn.2010-10.org.openstack:volume-076bb429-67fd-4c0c-9ddf-0dc7621a975a-lun-0'/>
<target dev='vdb' bus='virtio'/>
<serial>076bb429-67fd-4c0c-9ddf-0dc7621a975a</serial>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06'
function='0x0'/>
</disk>
I use centos7 and Liberty version of OpenStack.
Could anybody tell me how can I achieve it?
Thanks.
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