Snapshots on clustered LVM
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for storage. This enables us to do live migrations between the DRBD primary/secondary nodes.
We want to support iSCSI targergets in our HA enviroment. We are trying to see if we can use (c)lvm for that by creating a volume group of our iSCSI block devices and use that volume group on all nodes to create logical volumes. This seems to work fine if we handle locking etc properly and make sure we only activate the logical volumes on one node at a time. As long as we only have a volume active on one node snapshots seem to work fine also.
However, we run into problems when we want to perform a live migration of a running QEMU instance. In order to do a live migration we have to start a second similar QEMU on the node we want to migrate to and start a QEMU live migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During the live migration QEMU ensures that data is only written on one node (e.g. during the live migration data will be written on the source node, QEMU wil then pause the instance for a short while when copying the last data and will then continue the instance on the target node).
This use case works fine with a clustered LVM set-up except for snapshots. Changes are not saved in the snapshot when the logical volume is active on both nodes (as expected if the manual is correct: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).
If we are correct it means we can use lvm for as clustered "file system" but can't trust our snapshots to be 100% reliable if a volume group has been made active on more then one node. E.G. when doing a live migration between two nodes of a QEMU instance our snapshots become unreliable.
Are these conclusions correct? Is there a solution for this problem or is this simply a known limitation of clustered lvm without a work-around?
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Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a):
Quoted 28 lines
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for storage. This enables us to do live migrations between the DRBD primary/secondary nodes.
We want to support iSCSI targergets in our HA enviroment. We are trying to see if we can use (c)lvm for that by creating a volume group of our iSCSI block devices and use that volume group on all nodes to create logical volumes. This seems to work fine if we handle locking etc properly and make sure we only activate the logical volumes on one node at a time. As long as we only have a volume active on one node snapshots seem to work fine also.
However, we run into problems when we want to perform a live migration of a running QEMU instance. In order to do a live migration we have to start a second similar QEMU on the node we want to migrate to and start a QEMU live migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During the live migration QEMU ensures that data is only written on one node (e.g. during the live migration data will be written on the source node, QEMU wil then pause the instance for a short while when copying the last data and will then continue the instance on the target node).
This use case works fine with a clustered LVM set-up except for snapshots. Changes are not saved in the snapshot when the logical volume is active on both nodes (as expected if the manual is correct:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).
Quoted 7 lines
If we are correct it means we can use lvm for as clustered "file system" but can't trust our snapshots to be 100% reliable if a volume group has been made active on more then one node. E.G. when doing a live migration between two nodes of a QEMU instance our snapshots become unreliable.
Are these conclusions correct? Is there a solution for this problem or is this simply a known limitation of clustered lvm without a work-around?
Yes - snapshots are supported ONLY for exclusively actived volumes (means LV with snapshot is active only on a single node in cluster).
There is no dm target which would support clustered usage of snapshots.
Zdenek
Quoted 35 lines · Zdenek Kabelac
Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a):
Quoted 34 lines
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for storage. This enables us to do live migrations between the DRBD primary/secondary nodes.
We want to support iSCSI targergets in our HA enviroment. We are trying to see if we can use (c)lvm for that by creating a volume group of our iSCSI block devices and use that volume group on all nodes to create logical volumes. This seems to work fine if we handle locking etc properly and make sure we only activate the logical volumes on one node at a time. As long as we only have a volume active on one node snapshots seem to work fine also.
However, we run into problems when we want to perform a live migration of a running QEMU instance. In order to do a live migration we have to start a second similar QEMU on the node we want to migrate to and start a QEMU live migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During the live migration QEMU ensures that data is only written on one node (e.g. during the live migration data will be written on the source node, QEMU wil then pause the instance for a short while when copying the last data and will then continue the instance on the target node).
This use case works fine with a clustered LVM set-up except for snapshots. Changes are not saved in the snapshot when the logical volume is active on both nodes (as expected if the manual is correct:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).If we are correct it means we can use lvm for as clustered "file system" but can't trust our snapshots to be 100% reliable if a volume group has been made active on more then one node. E.G. when doing a live migration between two nodes of a QEMU instance our snapshots become unreliable.
Quoted 9 lines
Quoted 2 lines
Are these conclusions correct? Is there a solution for this problem or is this simply a known limitation of clustered lvm without a work-around?
Yes - snapshots are supported ONLY for exclusively actived volumes (means LV with snapshot is active only on a single node in cluster).
There is no dm target which would support clustered usage of snapshots.
Zdenek
Thanks for the confirmation. It's a pitty we can't get this done with LVM ... we will try to find an alternative.
Out of curiosity, how does a node know the volume is opened at another node? In our test set-up we don't use CLVM or anything (we are just testing), so there is no communication between the nodes. Is this done through meta data in the volume group / logical volume?
Dne 26.8.2015 v 14:22 Bram Klein Gunnewiek napsal(a):
Quoted 47 lines · Zdenek Kabelac
Quoted 38 lines
Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a):
Quoted 29 lines
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for storage. This enables us to do live migrations between the DRBD primary/secondary nodes.
We want to support iSCSI targergets in our HA enviroment. We are trying to see if we can use (c)lvm for that by creating a volume group of our iSCSI block devices and use that volume group on all nodes to create logical volumes. This seems to work fine if we handle locking etc properly and make sure we only activate the logical volumes on one node at a time. As long as we only have a volume active on one node snapshots seem to work fine also.
However, we run into problems when we want to perform a live migration of a running QEMU instance. In order to do a live migration we have to start a second similar QEMU on the node we want to migrate to and start a QEMU live migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During the live migration QEMU ensures that data is only written on one node (e.g. during the live migration data will be written on the source node, QEMU wil then pause the instance for a short while when copying the last data and will then continue the instance on the target node).
This use case works fine with a clustered LVM set-up except for snapshots. Changes are not saved in the snapshot when the logical volume is active on both nodes (as expected if the manual is correct:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).Quoted 7 lines
If we are correct it means we can use lvm for as clustered "file system" but can't trust our snapshots to be 100% reliable if a volume group has been made active on more then one node. E.G. when doing a live migration between two nodes of a QEMU instance our snapshots become unreliable.
Are these conclusions correct? Is there a solution for this problem or is this simply a known limitation of clustered lvm without a work-around?
Quoted 7 lines
Yes - snapshots are supported ONLY for exclusively actived volumes (means LV with snapshot is active only on a single node in cluster).
There is no dm target which would support clustered usage of snapshots.
Zdenek
Quoted 7 lines
Thanks for the confirmation. It's a pitty we can't get this done with LVM ... we will try to find an alternative.
Out of curiosity, how does a node know the volume is opened at another node? In our test set-up we don't use CLVM or anything (we are just testing), so there is no communication between the nodes. Is this done through meta data in the volume group / logical volume?
I've no idea what you are using then - I'm clearly talking only about lvm2 solution which is ATM based on clvmd usage (there is now integrated support for another locking manager - sanlock)
If you are using some other locking mechanism - it's then purely up-to-you to maintain integrity of the whole system - i.e. ensuring there are not multiple metadata writes from various nodes or where and how are the LVs activated.
Also there are already existing solutions for what you describe, but I assume you prefer your own home-brewed solution - but it's long journey ahead of you...
Zdenek
Quoted 3 lines · Zdenek Kabelac
Also there are already existing solutions for what you describe, but I assume you prefer your own home-brewed solution - but it's long journey ahead of you...
RHEV/ovirt is an existing solution that uses lvm on multiple hosts and does live migration. They have quite a bit of very specialized lvm code to do that right -- not typical lvm usage at all.
Quoted 66 lines · Bram Klein Gunnewiek
Quoted 57 lines · Zdenek Kabelac
Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a):
Quoted 52 lines
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for
storage. This enables us to do live migrations between the DRBD primary/secondary nodes.We want to support iSCSI targergets in our HA enviroment. We are trying to see
if we can use (c)lvm for that by creating a volume group of our iSCSI block
devices and use that volume group on all nodes to create logical volumes. This
seems to work fine if we handle locking etc properly and make sure we only
activate the logical volumes on one node at a time. As long as we only have a
volume active on one node snapshots seem to work fine also.However, we run into problems when we want to perform a live migration of a
running QEMU instance. In order to do a live migration we have to start a
second similar QEMU on the node we want to migrate to and start a QEMU live
migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During
the live migration QEMU ensures that data is only written on one node (e.g.
during the live migration data will be written on the source node, QEMU wil
then pause the instance for a short while when copying the last data and will
then continue the instance on the target node).This use case works fine with a clustered LVM set-up except for snapshots.
Changes are not saved in the snapshot when the logical volume is active on
both nodes (as expected if the manual is correct:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).If we are correct it means we can use lvm for as clustered "file system" but
can't trust our snapshots to be 100% reliable if a volume group has been made
active on more then one node. E.G. when doing a live migration between two
nodes of a QEMU instance our snapshots become unreliable.Quoted 3 lines
Are these conclusions correct? Is there a solution for this problem or is this
simply a known limitation of clustered lvm without a work-around?Quoted 7 lines
Yes - snapshots are supported ONLY for exclusively actived volumes (means LV with snapshot is active only on a single node in cluster).
There is no dm target which would support clustered usage of snapshots.
Zdenek
Quoted 7 lines
Thanks for the confirmation. It's a pitty we can't get this done with LVM ... we will try to find an alternative.
Out of curiosity, how does a node know the volume is opened at another node? In our test set-up we don't use CLVM or anything (we are just testing), so there is no communication between the nodes. Is this done through meta data in the volume group / logical volume?
Clustered LVM uses DLM. You can see which nodes are using a given lock space with 'dlm_tool ls'. When a node joins or leaves, it joins or leaves whatever lock spaces it's has resources using.
A nodes doesn't have to be actively using a resource, but if it's in a cluster, it needs to coordinate with the other nodes, even if just to say "I ACK the changes" or"I'm not using the resources" when coordinating locks.
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Quoted 5 lines · Bram Klein Gunnewiek
Currently we are using LVM as backing storage for our DRBD disks in HA set-ups. We use QEMU instances on our node's using (local) DRBD targets for storage. This enables us to do live migrations between the DRBD primary/secondary nodes.
Quoted 7 lines
We want to support iSCSI targergets in our HA enviroment. We are trying to see if we can use (c)lvm for that by creating a volume group of our iSCSI block devices and use that volume group on all nodes to create logical volumes. This seems to work fine if we handle locking etc properly and make sure we only activate the logical volumes on one node at a time. As long as we only have a volume active on one node snapshots seem to work fine also.
DRBD, like an iSCSI LUN, is just another block device to LVM. So I see no reason why clvmd won't work just fine. Main advantage is that you can scale iscsi to 3+ nodes, but you lose data being replicated unless you have a very nice SAN.
Once the LV is visible on all nodes though, it's up to you to make sure they're used by apps/fses that understand clustering. I use clustered LVs to back gfs2 and to back VMs (LV dedicated to a VM, and the cluster resource manager ensures that a VM is only on one node at a time).
Quoted 10 lines
However, we run into problems when we want to perform a live migration of a running QEMU instance. In order to do a live migration we have to start a second similar QEMU on the node we want to migrate to and start a QEMU live migration. In order for us to do that we have to make the logical volume active on the target node otherwise we can't start the QEMU instance. During the live migration QEMU ensures that data is only written on one node (e.g. during the live migration data will be written on the source node, QEMU wil then pause the instance for a short while when copying the last data and will then continue the instance on the target node).
If you're using clustered LVM, live migration will work just fine. This is exactly what I do. The LV will need to be ACTIVE on both nodes though.
Quoted 4 lines
This use case works fine with a clustered LVM set-up except for snapshots. Changes are not saved in the snapshot when the logical volume is active on both nodes (as expected if the manual is correct:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes).
Note that your link is very old, for RHEL 5.
Snapshotting is a problem. As Zdenek said, you have to set the other nodes to inactive and then set the current host node's LV to 'exclusive'. Trick I found though was that you can't mark it as exclusive while it's ACTIVE, and you can't make the LV inactive while it's hosting a VM... So in practical terms, snapshotting clustered LVs is not feasible.
Quoted 4 lines
If we are correct it means we can use lvm for as clustered "file system" but can't trust our snapshots to be 100% reliable if a volume group has been made active on more then one node. E.G. when doing a live migration between two nodes of a QEMU instance our snapshots become unreliable.
You can never trust a snapshot 100%; It doesn't capture information in the VM's memory. So at best, using a snapshot to recover is like recovering from a sudden power loss. It's then up to your apps and OS to recover, and that's not always the case with many DBs, unless they're carefully configured.
This is the core reason why our company won't support snapshots at all. It gives people a false sense of having good backups.
Quoted 2 lines
Are these conclusions correct? Is there a solution for this problem or is this simply a known limitation of clustered lvm without a work-around?
Clustered LVs over a SAN-backed PV will work perfectly fine for live migrations. Snapshots are not feasible though, and not recommended in any case.
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Digimer Papers and Projects: https://alteeve.ca/w/ What if the cure for cancer is trapped in the mind of a person without access to education?