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<div class="moz-cite-prefix">One major disadvantage is lack of
multipath support.<br>
<br>
Multipath is still done outside of qemu and there is no native
multipath support inside of qemu from what I can tell. Another<br>
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.<br>
<br>
Do you have some actual numbers to prove that host based attaches
passed into libvirt are slower than QEMU direct attaches?<br>
<br>
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.<br>
<br>
Walt<br>
<br>
<br>
On 06/15/2016 04:59 PM, Preston L. Bannister wrote:<br>
</div>
<blockquote
cite="mid:CA+R0Njb8wztW5cJrMf9BN6y=QbUqh6gMaihQ0PHL2_1x9QHt-Q@mail.gmail.com"
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<div>QEMU has the ability to directly connect to iSCSI volumes.
Running the iSCSI connections through the nova-compute host
*seems* somewhat inefficient. </div>
<div><br>
</div>
<div>There is a spec/blueprint and implementation that landed in
Kilo:</div>
<div><br>
</div>
<div><a moz-do-not-send="true"
href="https://specs.openstack.org/openstack/nova-specs/specs/kilo/implemented/qemu-built-in-iscsi-initiator.html">https://specs.openstack.org/openstack/nova-specs/specs/kilo/implemented/qemu-built-in-iscsi-initiator.html</a><br>
</div>
<div><a moz-do-not-send="true"
href="https://blueprints.launchpad.net/nova/+spec/qemu-built-in-iscsi-initiator">https://blueprints.launchpad.net/nova/+spec/qemu-built-in-iscsi-initiator</a><br>
</div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>Looking for a general sense where this is headed. (If
anyone knows...)</div>
<div><br>
</div>
<div>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?</div>
<div><br>
</div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>As our customers are (so far) only asking about virtual
machine backup. I long ago settled on (2) as most promising. </div>
<div><br>
</div>
<div>What I cannot clearly determine is where (1) will fail.
Will all iSCSI volumes connected to QEMU instances eventually
become directly connected? </div>
<div><br>
</div>
<div><br>
</div>
<div>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.)</div>
<div><br>
</div>
<div>To answer Xiao's question, can a site configure their cloud
to *always* directly connect iSCSI volumes to QEMU?</div>
<div><br>
</div>
<br>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Feb 16, 2016 at 4:54 AM, Xiao
Ma (xima2) <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:xima2@cisco.com" target="_blank">xima2@cisco.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">
<div style="word-wrap:break-word">
<div>
<div
style="color:rgb(0,0,0);letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;word-wrap:break-word">
<div>
<div>Hi, All</div>
<div><br>
</div>
<div>I want to make the qemu communicate with
iscsi target using libiscsi directly, and I </div>
<div>followed <a moz-do-not-send="true"
href="https://review.openstack.org/#/c/135854/"
target="_blank">https://review.openstack.org/#/c/135854/</a> to
add </div>
<div><span
style="background-color:rgb(255,255,255)">'</span><span
style="background-color:rgb(255,255,255)"><font
face="monospace" size="2"><span style="white-space:pre-wrap">volume_drivers
= iscsi=nova.virt.libvirt.volume.LibvirtNetVolumeDriver’</span></font> in </span>nova.conf</div>
<div> and then restarted nova services and cinder
services, but still the volume configuration of
vm is as bellow:</div>
<div><br>
</div>
<div>
<div> <disk type='block' device='disk'></div>
<div> <driver name='qemu' type='raw'
cache='none'/></div>
<div> <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'/></div>
<div> <target dev='vdb'
bus='virtio'/></div>
<div>
<serial>076bb429-67fd-4c0c-9ddf-0dc7621a975a</serial></div>
<div> <address type='pci'
domain='0x0000' bus='0x00' slot='0x06'
function='0x0'/></div>
<div> </disk></div>
<div><br>
</div>
<div><br>
</div>
<div>I use centos7 and Liberty version of
OpenStack.</div>
<div>Could anybody tell me how can I achieve it?</div>
<div><br>
</div>
<div><br>
</div>
</div>
<div>Thanks.</div>
</div>
</div>
</div>
</div>
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