10:57 p.m.
I apologize if this is a duplicate. I submitted it before I was a list
member and it looks like it went into limbo.
Dear Moderator, please discard the original if it is sitll in a moderation
queue.
Hello,
I've spent a few days googling and reading documentation, but I'm looking for
clarification and advise on setting up KVM/libvirt with shared storage.
I have 2 (for now) Ubuntu Karmic systems with KVM/virsh/virt-manager set up and
running.
I have a storage server that can do NFS/iSCSI/samba/ etc.
I am trying to figure out the best way to set things up so that I can run
several (eventually) production linux guests (mostly debian stable) on the 2
KVM host systems and be able to migrate them when maintenance is required on
their hosts.
Performance is of medium importance, stability/availibilty is pretty high
priority.
A common opinion seems to be that using LVs to hold disk images gives possibly
the best IO performance, followed by raw, then qcow2.
Would you agree or disagree with this? What evidence can you provide?
I am also rather confused about using shared storage, and what options can be
combined with this.
I have succesfully made an iscsi device available to libvirt/virsh/virt-manager
via an XML file + pool-define + pool-start. However the documentation states
that while you can create pools through libvirt, volumes have to be
pre-allocated:
http://libvirt.org/storage.html
"Volumes must be pre-allocated on the iSCSI server, and cannot be created via
the libvirt APIs."
I'm very unclear on what this means in general, and specifically how you
preallocate the the Volumes.
When I make a iscsi pool available via something like this:
<pool type="iscsi">
<name>virtimages</name>
<source>
<host name="iscsi.example.com"/>
<device path="demo-target"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool>
In virt-manger I can see it as a pool, but when I try to select it for where to
create an image, it uses the pool AS A Volume (I think).
That brings me to my next question/misunderstanding...
If you are using shared storage (via NFS or iSCSI) does that also mean you MUST
use a file based image rather than an LVM LV? NFS makes directories available,
not devices. You can make an unformated "raw" LV available via iSCSI, but its
not seen on the far side as a LV but instead as a scsi disk which you need to
partition. You can make a PV>VG>LV out of that and then make your new LV
available to libvirt/KVM but then your stacking a heck of a lot of
LVM/LVs/partion tables up which seems pretty dubious. I'm also not sure that
the stack would be visable/available to the second system using that iSCSI
device. (I'm pretty new to using iSCSI as well.)
Redhat provides pretty good documentation on doing shared storage/live
migration for NFS:
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Virtua...
But unfortunately the section on Shared Storage with iSCSI is a bit lacking:
"9.1. Using iSCSI for storing guests
This section covers using iSCSI-based devices to store virtualized guests." and
thats it.
For maybe 6-10 guests, should I simply just be using NFS? Is its performance
that much worse than iSCSI for this task?
So since KVM/libvirt is getting used quite often in production now - but some
simple things are not making any sense to me - I'm guessing that I have some
really basic misunderstandings of how to do shared storage / Pools/ Volumes for
migration.
Would one of the kind readers of this least be willing to correct me?
Please CC to my original email address as well as I am not yet a regular
subscriber.
Thanks again!
David.
4:18 a.m.
On 05/03/2010 10:57 AM, David Ehle wrote:
I've spent a few days googling and reading documentation, but
I'm looking for
clarification and advise on setting up KVM/libvirt with shared storage.
I, too, have been looking at these kinds of setups in the last few
weeks, and I've found myself confused about the stack more than once.
The documentation is very scattered and incomplete in key areas (as you
pointed out with the RHEL docs on shared storage), which makes it tough
on a new user. Here's what I can share, I hope this encourages others
to join in and help us both out.
I am trying to figure out the best way to set things up so that I can
run
several (eventually) production linux guests (mostly debian stable) on the 2
KVM host systems and be able to migrate them when maintenance is required on
their hosts.
From the small testing I've done, making an iscsi based storage pool
and using it to store the VMs between the two KVM hosts.
A common opinion seems to be that using LVs to hold disk images gives
possibly
the best IO performance, followed by raw, then qcow2.
Would you agree or disagree with this? What evidence can you provide?
That's probably right, but I think those are more relevant when you are
using non-shared storage, like images on your local disk or perhaps on a
shared mount.
I have succesfully made an iscsi device available to
libvirt/virsh/virt-manager
via an XML file + pool-define + pool-start. However the documentation states
that while you can create pools through libvirt, volumes have to be
pre-allocated:
http://libvirt.org/storage.html
"Volumes must be pre-allocated on the iSCSI server, and cannot be created via
the libvirt APIs."
I'm very unclear on what this means in general, and specifically how you
preallocate the the Volumes.
I believe this means you will need to create the LUNs on the storage and
present them to libvirt, but libvirt cannot talk back to the storage
system and create the targets/LUNs for you as you try to spin up new VMs.
If you are using shared storage (via NFS or iSCSI) does that also
mean you MUST
use a file based image rather than an LVM LV?
No, you can use iscsi and treat the storage as a raw disk.
Redhat provides pretty good documentation on doing shared
storage/live
migration for NFS:
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Virtua...
But unfortunately the section on Shared Storage with iSCSI is a bit lacking:
"9.1. Using iSCSI for storing guests
This section covers using iSCSI-based devices to store virtualized guests." and
thats it.
I know! How frustrating! Even the RHEL6 beta document is missing those
sections...
Here's what I've documented for my initial test run. I hope this helps
you get started, please note that I changed the IPs and iqn's to prevent
sharing private data on the list.
1. Create a LUN on your shared storage system & get the iqn.
iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84
2. From the KVM host, scan for the newly created target
# iscsiadm -m discovery -t sendtargets -p <shared storage FQDN>
1.2.3.4:3260,2 iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84
There it is, *84.
3. Write the XML config describing the storage we will add to the
storage pool, using the iqn from step 2.
<pool type="iscsi">
<name>lun1</name>
<source>
<host name="storage hostname"/>
<device
path="iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool>
4. Import that XML to define the pool:
# virsh pool-define /tmp/lun1.xml
Pool lun1 defined from lun1.xml
5. Verify:
# virsh pool-info lun1
Name: lun1
UUID: 701a1401-547a-08da-5f14-befea84778d9
State: inactive
6. Start the pool so we can use it
# virsh pool-start lun1
Pool lun1 started
7. List the volume names in the new pool so we'll know what to pass
along to virt-install to use it:
# virsh vol-list lun1
Name Path
-----------------------------------------
10.0.0.0
/dev/disk/by-path/ip-1.2.3.4:3260-iscsi-iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84-lun-0
virsh # vol-info --pool lun1 10.0.0.0
Name: 10.0.0.0
Type: block
Capacity: 20.00 GB
Allocation: 20.00 GB
8. Install the VM with virt-install and use "--disk
vol=lun1/10.0.0.0,size=20" to access the storage
[root@rvrt010d ~]# virt-install <all of your options>
--disk vol=lun1/10.0.0.0,size=20
After that's up and running, your VM will use the shared storage as if
it is a local disk.
9. If you repeat steps 2-6 on your other KVM host, the storage will be
shared between both hosts. Now you are setup for migration, either
offline or live.
# virsh migrate VM qemu+ssh://other.kvm.host/system
For maybe 6-10 guests, should I simply just be using NFS? Is its
performance
that much worse than iSCSI for this task?
This totally depends on your network and hardware, but here's how a dd
test over NFS and iSCSI compared in my setup:
iSCSI
run throughput
01 24.5 MB/s
02 25.7 MB/s
03 52.4 MB/s
04 56.5 MB/s
05 57.7 MB/s
06 56.2 MB/s
07 51.5 MB/s
08 57.8 MB/s
09 57.3 MB/s
10 55.8 MB/s
NFS
run throughput
01 34.5 MB/s
02 35.2 MB/s
03 36.2 MB/s
04 41.2 MB/s
05 39.2 MB/s
06 39.2 MB/s
07 36.2 MB/s
08 34.8 MB/s
09 36.7 MB/s
10 40.4 MB/s
I hope this helps, and I hope that someone corrects any errors I may
have made. :)
Tom
4:19 a.m.
On 05/03/2010 10:57 AM, David Ehle wrote:
I've spent a few days googling and reading documentation, but
I'm looking for
clarification and advise on setting up KVM/libvirt with shared storage.
I, too, have been looking at these kinds of setups in the last few
weeks, and I've found myself confused about the stack more than once.
The documentation is very scattered and incomplete in key areas (as you
pointed out with the RHEL docs on shared storage), which makes it tough
on a new user. Here's what I can share, I hope this encourages others
to join in and help us both out.
I am trying to figure out the best way to set things up so that I can
run
several (eventually) production linux guests (mostly debian stable) on the 2
KVM host systems and be able to migrate them when maintenance is required on
their hosts.
From the small testing I've done, making an iscsi based storage pool
and using it to store the VMs between the two KVM hosts.
A common opinion seems to be that using LVs to hold disk images gives
possibly
the best IO performance, followed by raw, then qcow2.
Would you agree or disagree with this? What evidence can you provide?
That's probably right, but I think those are more relevant when you are
using non-shared storage, like images on your local disk or perhaps on a
shared mount.
I have succesfully made an iscsi device available to
libvirt/virsh/virt-manager
via an XML file + pool-define + pool-start. However the documentation states
that while you can create pools through libvirt, volumes have to be
pre-allocated:
http://libvirt.org/storage.html
"Volumes must be pre-allocated on the iSCSI server, and cannot be created via
the libvirt APIs."
I'm very unclear on what this means in general, and specifically how you
preallocate the the Volumes.
I believe this means you will need to create the LUNs on the storage and
present them to libvirt, but libvirt cannot talk back to the storage
system and create the targets/LUNs for you as you try to spin up new VMs.
If you are using shared storage (via NFS or iSCSI) does that also
mean you MUST
use a file based image rather than an LVM LV?
No, you can use iscsi and treat the storage as a raw disk.
Redhat provides pretty good documentation on doing shared
storage/live
migration for NFS:
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Virtua...
But unfortunately the section on Shared Storage with iSCSI is a bit lacking:
"9.1. Using iSCSI for storing guests
This section covers using iSCSI-based devices to store virtualized guests." and
thats it.
I know! How frustrating! Even the RHEL6 beta document is missing those
sections...
Here's what I've documented for my initial test run. I hope this helps
you get started, please note that I changed the IPs and iqn's to prevent
sharing private data on the list.
1. Create a LUN on your shared storage system & get the iqn.
iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84
2. From the KVM host, scan for the newly created target
# iscsiadm -m discovery -t sendtargets -p <shared storage FQDN>
1.2.3.4:3260,2 iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84
There it is, *84.
3. Write the XML config describing the storage we will add to the
storage pool, using the iqn from step 2.
<pool type="iscsi">
<name>lun1</name>
<source>
<host name="storage hostname"/>
<device
path="iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84"/>
</source>
<target>
<path>/dev/disk/by-path</path>
</target>
</pool>
4. Import that XML to define the pool:
# virsh pool-define /tmp/lun1.xml
Pool lun1 defined from lun1.xml
5. Verify:
# virsh pool-info lun1
Name: lun1
UUID: 701a1401-547a-08da-5f14-befea84778d9
State: inactive
6. Start the pool so we can use it
# virsh pool-start lun1
Pool lun1 started
7. List the volume names in the new pool so we'll know what to pass
along to virt-install to use it:
# virsh vol-list lun1
Name Path
-----------------------------------------
10.0.0.0
/dev/disk/by-path/ip-1.2.3.4:3260-iscsi-iqn.long.long.long:a6c98a7e-0275-c020-efae-a386dfd46b84-lun-0
virsh # vol-info --pool lun1 10.0.0.0
Name: 10.0.0.0
Type: block
Capacity: 20.00 GB
Allocation: 20.00 GB
8. Install the VM with virt-install and use "--disk
vol=lun1/10.0.0.0,size=20" to access the storage
[root@rvrt010d ~]# virt-install <all of your options>
--disk vol=lun1/10.0.0.0,size=20
After that's up and running, your VM will use the shared storage as if
it is a local disk.
9. If you repeat steps 2-6 on your other KVM host, the storage will be
shared between both hosts. Now you are setup for migration, either
offline or live.
# virsh migrate VM qemu+ssh://other.kvm.host/system
For maybe 6-10 guests, should I simply just be using NFS? Is its
performance
that much worse than iSCSI for this task?
This totally depends on your network and hardware, but here's how a dd
test over NFS and iSCSI compared in my setup:
iSCSI
run throughput
01 24.5 MB/s
02 25.7 MB/s
03 52.4 MB/s
04 56.5 MB/s
05 57.7 MB/s
06 56.2 MB/s
07 51.5 MB/s
08 57.8 MB/s
09 57.3 MB/s
10 55.8 MB/s
NFS
run throughput
01 34.5 MB/s
02 35.2 MB/s
03 36.2 MB/s
04 41.2 MB/s
05 39.2 MB/s
06 39.2 MB/s
07 36.2 MB/s
08 34.8 MB/s
09 36.7 MB/s
10 40.4 MB/s
I hope this helps, and I hope that someone corrects any errors I may
have made. :)
Tom
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