From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [10.34.130.168] (dhcp130-168.brq.redhat.com [10.34.130.168]) by int-mx14.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id t7QAxetJ017419 for ; Wed, 26 Aug 2015 06:59:41 -0400 References: <55DC3EBF.4030703@shockmedia.nl> From: Zdenek Kabelac Message-ID: <55DD9C1C.9070105@redhat.com> Date: Wed, 26 Aug 2015 12:59:40 +0200 MIME-Version: 1.0 In-Reply-To: <55DC3EBF.4030703@shockmedia.nl> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Snapshots on clustered LVM Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: LVM general discussion and development Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a): > 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? 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