Provisioning KVM virtual machines on iSCSI the hard way (Part 1 of 2)

Posted: May 5th, 2010 | Filed under: libvirt, Virt Tools | Tags: , | 2 Comments »

The previous articles showed how to provision a guest on iSCSI the nice & easy way using a QNAP NAS and virt-manager. This article and the one that follows, will show how to provision a guest on iSCSI the “hard way”, using the low level command line tools tgtadm, virsh and virt-install. The iSCSI server in this case is going to be provided by a guest running Fedora 13, x86_64.

Enabling the iSCSI target service

The first task is to install and enable the iSCSI target service. On Fedora and RHEL servers, the iSCSI target service is provided by the ‘scsi-target-utils’ RPM package, so install that now and set the service to start on boot

# yum install scsi-target-utils
# chkconfig tgtd on
# service tgtd start

Allocating storage for the LUNs

The  Linux SCSI target service does not care whether the LUNs exported are backed by plain files, LVM volumes or raw block devices, though obviously there is some performance overhead from introducing the LVM and/or filesystem layers as compared to block devices. Since the guest providing the iSCSI service in this example has no spare block device or LVM space, raw files will have to be used. In this example, two LUNs will be created one thin provisioned (aka sparse file) 10 GB LUN and one fully allocated 500 MB LUN

# mkdir -p /var/lib/tgtd/kvmguests
# dd if=/dev/zero of=/var/lib/tgtd/kvmguests/rhel6x86_64.img bs=1M seek=10240 count=0
# dd if=/dev/zero of=/var/lib/tgtd/kvmguests/shareddata.img bs=1M count=512
# restorecon -R /var/lib/tgtd

Exporting an iSCSI target and LUNs (the manual way)

Historically, you had to invoke a series of tgtadm commands to setup the iSCSI target and LUNs and then add them to /etc/rc.d/rc.sysinit to make sure they run on every boot. This is true of RHEL5 vintage scsi-target-utils at least. If you have a more recent version circa Fedora 13 / RHEL-6 there is finally a nice configuration file to handle this setup, so those lucky readers can skip ahead. The first step is to add a target, for this the adorable IQNs make a re-appearance

# tgtadm --lld iscsi --op new --mode target --tid 1 --targetname iqn.2004-04.fedora:fedora13:iscsi.kvmguests

Next step is to associate the storage volumes, just created, with LUNs in the iSCSI target.

# tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1 --backing-store /var/lib/tgtd/kvmguests/rhel6x86_64.img
# tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 2 --backing-store /var/lib/tgtd/kvmguests/shareddata.img

To confirm that all went to plan, query the iSCSI target setup

# tgtadm  --lld iscsi --op show --mode target
Target 1: iqn.2004-04.fedora:fedora13:iscsi.kvmguests
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB
            Online: Yes
            Removable media: No
            Backing store type: rdwr
            Backing store path: None
        LUN: 1
            Type: disk
            SCSI ID: IET     00010001
            SCSI SN: beaf11
            Size: 10737 MB
            Online: Yes
            Removable media: No
            Backing store type: rdwr
            Backing store path: /var/lib/tgtd/kvmguests/rhel6x86_64.img
        LUN: 2
            Type: disk
            SCSI ID: IET     00010002
            SCSI SN: beaf12
            Size: 537 MB
            Online: Yes
            Removable media: No
            Backing store type: rdwr
            Backing store path: /var/lib/tgtd/kvmguests/shareddata.img
    Account information:
    ACL information:

Finally, allow client access to the target. This example allows access to all clients without any authentication.

# tgtadm --lld iscsi --op bind --mode target --tid 1 --initiator-address ALL

Exporting an iSCSI target and LUNs (with a config file)

As mentioned earlier, modern versions of scsi-target-utils now include a configuration file for setting up targets and LUNs. The master configuration is /etc/tgt/targets.conf and is full of example configurations. To replicate the manual setup from above requires adding a configuration block that looks like this

<target iqn.2004-04.fedora:fedora13:iscsi.kvmguests>
backing-store /var/lib/tgtd/kvmguests/rhel6x86_64.img
backing-store /var/lib/tgtd/kvmguests/shareddata.img

With the configuration update, load it into the iSCSI target daemon

#tgt-admin --execute

Two common mistakes

The two most likely places to trip up when configuring the iSCSI target are SELinux and iptables. If adding plain files as LUNs in an iSCSI target, make sure the files are labelled suitably with system_u:object_r:tgtd_var_lib_t:s0. For iptables, ensure that port 3260 is open.

That is a very quick guide to setting up an iSCSI target on Fedora 13. The next step is to switch back to the virtualization host and provision a new guest using iSCSI for its virtual disk. This is covered in Part II