Storage – LeftAsExercise

OpenStack Cinder – architecture and installation

Having looked at the foundations of the storage technology that Cinder uses in the previous posts, we are now ready to explore the basic architecture of Cinder and install Cinder in our playground.

Cinder architecture

Essentially, Cinder consists of three main components which are running as independent processes and typically on different nodes.

First, there is the Cinder API server cinder-api. This is a WSGI-server running inside of Apache2. As the name suggests, cinder-api is responsible for accepting and processing REST API requests from users and other components of OpenStack and typically runs on a controller node.

Then, there is cinder-volume, the Cinder volume manager. This component is running on each node to which storage is attached (“storage node”) and is resonsible for managing this storage, i.e. preparing, maintaining and deleting virtual volumes and exporting these volumes so that they can be used by a compute node. And finally, Cinder comes with its own scheduler, which directs requests to create a virtual volume to an appropriate storage node.

Of course, Cinder also has its own database and communicates with other components of OpenStack, for example with Keystone to authenticate requests and with Glance to be able to create volumes from images.

Cinder van use a variety of different storage backends, ranging from LVM managing local storage directly attached to a storage node over other standards like NFS and open source solutions like Ceph to vendor-specific drivers like Dell, Huawei, NetApp or Oracle – see the official support matrix for a full list. This is achieved by moving low-level access to the actual volumes into a volume driver. Similarly, Cinder uses various technologies to connect the virtual volumes to the compute node on which an instance that wants to use it is running, using a so-called target driver.

To better understand how Cinder operates, let us take a look at one specific use case – creating a volume and attaching to an instance. We will go in more detail on this and similar use cases in the next post, but what essentially happens is the following:

A user (for instance an administrator) sends a request to the Cinder API server to create a logical volume
The API server asks the scheduler to determine a storage node with sufficient capacity
The request is forwarded to the volume manager on the target node
The volume manager uses a volume driver to create a logical volume. With the default settings, this will be LVM, for which a volume group and underlying physical volumes have been created during installation. LVM will thzen create a new logical volume
Next, the administrator attaches the volume to an instance using another API request
Now, the target driver is invoked which sets up an iSCSI target on the storage node and a LUN pointing to the logical volume
The storage node informs the compute node about the parameters (portal IP and port, target name) that are needed to access this target
The Nova compute agent on the compute node invokes an iSCSI initiator which maps the iSCSI target into the local file system
Finally, the Nova compute agent asks the virtual machine driver (libvirt in our case) to attach this locally mapped device to the instance

Installation steps

After this short summary of the high-level architecture of Cinder, let us move and try to understand how the installation process looks like. Again, the installation follows the standard pattern that we have now seen so many times.

Create a database for Cinder and prepare a database user
Add user, services and endpoints in Keystone
Install the Cinder packages on the controller and the storage nodes
Modify the configuration files as needed

In addition to these standard steps, there are a few points specific for Cinder. First, as sketched above, Cinder uses LVM to manage virtual volumes on the storage nodes. We therefore need to perform a basic setup of LVM on each storage node, i.e. we need to create physical volumes and a volume group. Second, every compute node will need to act as iSCSI initiator and therefore needs a valid iSCSI initiator name. The Ubuntu distribution that we use already contains the Open-iSCSI package, which maintains an initiator name in the file /etc/iscsi/initiatorname.iscsi. However, as this name is supposed to be unique, it is not set in the Ubuntu Vagrant image and we need to do this once during the installation by running the script /lib/open-iscsi/startup-checks.sh as root.

There is an additional point that we need to observe which is also not mentioned in the official installation guide for the Stein release. When installing Cinder, you have to define which network interface Cinder will use for the iSCSI traffic. In production, you would probably want to use a separate storage network, but for our setup, we use the management network. According to the installation guide, it should be sufficient to set the configuration item my_ip, but in reality, this did not work for me and I had to set the item target_ip_address on the storage nodes.

Lab13: running and testing the Cinder installation

Time to try this out and set up a lab for that. The first thing we need to do is to modify our Vagrantfile to add a storage node. In order to reduce memory consumption a bit, we instead move from having two computes to one compute node only, so that our new setup looks as follows.

As mentioned above, we will not use a dedicated storage network, but send the iSCSI traffic over the management network so that our network setup remains essentially unchanged. To bring up this scenario and to run the Cinder installation plus a demo setup do the following.

git clone https://github.com/christianb93/openstack-labs
cd openstack-labs/Lab13
vagrant up
ansible-playbook -i hosts.ini site.yaml
ansible-playbook -i hosts.ini demo.yaml

When all this completes, we can run some tests. First, verify that all storage nodes are up and running. For that purpose, log into the controller node and use the OpenStack CLI to retrieve a list of all recognized storage nodes.

vagrant ssh controller
source admin-openrc
openstack volume service list

The output that you get should look similar to the sample output below.

+------------------+-------------+------+---------+-------+----------------------------+
| Binary           | Host        | Zone | Status  | State | Updated At                 |
+------------------+-------------+------+---------+-------+----------------------------+
| cinder-scheduler | controller  | nova | enabled | up    | 2020-01-01T14:03:14.000000 |
| cinder-volume    | storage@lvm | nova | enabled | up    | 2020-01-01T14:03:14.000000 |
+------------------+-------------+------+---------+-------+----------------------------+

We see that there are two services that have been detected. First, there is the Cinder scheduler running on the controller node, and then, there is the Cinder volume manager on the storage node. Both are “up”, which is promising.

Next, let us try to create a volume and attach it to one of our test instances, say demo-instance-3. For this to work, you need to be on the network node (as we will have to establish connectivity to the instance).

vagrant ssh network
source demo-openrc
openstack volume create \
  --size=1 \
  demo-volume
openstack server add volume \
  demo-instance-3 \
  demo-volume
openstack server show demo-instance-3
openstack server ssh \
  --identity demo-key \
  --login cirros \
  --private demo-instance-3

You should now be inside the instance. When you run lsblk, you should find a new block device /dev/vdb being added. So our installation seems to work!

Having a working testbed, we are now ready to dive a little deeper into the inner workings of Cinder. In the next post, we will try to understand some of the common use cases in a bit more detail.

OpenStack Cinder foundations – building logical volumes and snapshots with LVM

When you want to build a volume service for a cloud platform, you need to find a way to quickly create and remove block devices on your compute nodes. We could of course use loopback devices for this, but this is slow, as every operation goes through the file system. A logical volume manager might be a better alternative. Today, we will investigate the logical volume manager that Cinder actually uses – Linux LVM2.

The Linux logical volume manager – some basic terms

In this section, we will briefly explain some of the key concepts of the Linux logical volume manager (LVM2). First, there are of course physical devices. These are ordinary block devices that the LVM will completely manage, or partitions on block devices. Technically, even though these devices are called physical devices in this context, these devices can themselves be virtual devices, which happens for instance if you run LVM on top of a software RAID. Logically,
the physical devices are divided further into physical extents. An extent the smallest unit of storage that LVM manages.

On the second layer, LVM now bundles one or several physical devices into a volume group. On top of that volume group, you can now create logical devices. These logical devices can be thought of as being divided into logical extents. LVM maps these logical extents to physical extents of the underlying volume group. Thus, a logical device is essentially a collection of physical extents of the underlying volume group which are presented to a user as a logical block device. On top of these logical volumes, you can then create file systems as usual.

Why would you want to do this? One obvious advantage is again based on the idea of pooling. A logical volume essentially pools the storage capacity of the underlying physical devices and LVM can dynamically assign space to logical devices. If a logical device starts to fill up while other logical devices are still mostly empty, an administrator can simply reallocate capacity between the logical devices without having to change the physical configuration of the system.

Another use case is virtualization. Given that there is sufficiently storage in your logical volume group, you can dynamically create new logical devices with a simple command, which can for instance be used to automatically provision volumes for cloud instances – this is how Cinder leverages the LVM as we will see later on.

Looking at this, you might be reminded of a RAID controller which also manages physical devices and presents their capacity as virtual RAID volumes. It is important to understand that LVM is not (primarily) a RAID manager. In fact, newer versions of LVM also offer RAID functionality (more on this below), but this it not its primary purpose.

Another useful functionality that LVM offers is a snapshot. When you create a snapshot, LVM will not simply create a physical copy. Instead, it will start to mark blocks which are changed after the snapshot has been taken as changed and only copy those blocks to a different location. This makes using the snapshot functionality very efficient.

Lab12: installing and using LVM

Let us now try to see how LVM works in practice. First, we need a machine with a couple of unused block devices. As it is unlikely that you have some spare disks lying around under your desk, we will again use a virtual machine for that purpose. So bring up our test machine and log into it using the following commands (assuming that you have gone through the basic setup steps in the the first post in this series).

git clone https://github.com/christianb93/openstack-labs
cd openstack-labs/Lab12
vagrant up
vagrant ssh box

When you now run lsblk inside the machine, you should see two additional devices /dev/sdc and /dev/sdd which are both unmounted and have a capacity of 5 GB each.

As a first step, let us now prepare these physical volumes for use with LVM. This is done using the pvcreate utility. WARNING: if you accidentally run this outside of the VM, it will render the device unusable!

sudo pvcreate /dev/sdc
sudo pvcreate /dev/sdd

What is this command actually doing? To understand this, let us first use pvscan to print a list of all physical volumes on the system which LVM knows.

sudo pvscan -u

You will see a list of two volumes, and after each volume, LVM will print a UUID for this volume. Now let us see what LVM has actually written on the volume.

sudo dd if=/dev/sdc \
  bs=1024 \
  count=10 \
  | hexdump -C

In the output, you will see that LVM has written some sort of signature onto the device, containing some binary information and the UUID of the device. In fact, this is how LVM stores state and is able to recognize a volume even if it has been moved to a different point in /dev.

Now we can build our first volume group. For that purpose, we use the command vgcreate and specify the name of the volume group and a list of physical devices that the volume group should contain.

sudo vgcreate test_vg /dev/sdc /dev/sdd

If you now repeat the dump above, you will see that LVM has again written some additional data on the device, we find the name of the newly created volume group and even a JSON representation of the physical volumes in the volume group.

Let us now print out a bit more information on the system using the lvm shell. So run

sudo lvm

to start the shell and then type fullreport to get a description of the current configuration. It is instructive to play a bit with the shell, use help to get a list of available commands and exit to exit the shell when you are done.

Finally, it is now time to create a few logical volumes. Our entire volume group has 10 GB available. We will create three logical volumes which in total consume 6 GB.

for i in {1..3}; do
  sudo lvcreate \
    --size 2G \
    test_vg \
    --type linear \
   --name lv0$i
done
sudo lvscan

The last command will print a list of all logical volumes on the system and should display the three logical volumes that we have just created. If you now again create a full report using lvm, you will find these three devices and a table that indicates how the logical extends are distributed across the various physical devices.

Behind the scenes, LVM uses the Linux device mapper kernel module, and in fact, each device that we create is displayed in the /dev tree at three different points. First, LVM exposes the logical volumes at a location built according to the scheme

/dev/volume group name/logical volume name

In our example, the first volume, for instance, is located at /dev/test_vg/lv01. This, however, is only a link to the device /dev/dm-0, indicating that it is created by the device mapper. Finally, a second link is created in /dev/mapper.

The LVM metadata daemon

We have said above that LVM stores its state on the physical devices. This, however, is only a part of the story, as it would imply that whenever we use one of the tools introduced above, we have to scan all devices, which is slow and might interfere with other read or write access to the device.

For that reason, LVM comes with a metadata daemon, running as lvmetad in the background as a systemctl service. This daemon maintains a cache of the LVM metadata that a command like lvscan will typically use (you can see this if you try to run such a command as non-root, which will cause an error message while the tool is trying to connect to the daemon via a Unix domain socket).

The metadata daemon is also involved when devices are added (hotplug), removed, or changed. If, for example, a physical volume comes up, a Linux kernel mechanism known as udev informs LVM about this event, and when a volume group is complete, all logical volumes based on it are automatically activated (see the comment on use_lvmetad in the configuration file /etc/lvm/lvm.conf).

It is interesting to take a look at the udev ruleset that LVM creates for this purpose (you will find these rules in the LVM-related files in /lib/udev/rules.d, in my distribution, these are the files with the numbers 56 and 69). In the rules file 69-lvm-metad.rules, for instance, you will find a rule that invokes (via systemd dependencies) a pvscan every time a physical device is added which will update the cache maintained by the metadata daemon (see also this man-page for a bit more background on the various options that you have to activate logical LVM devices at boot-time).

However, there is one problem with this type of scan that should be mentioned. Suppose, in our scenario, someone exports our logical device /dev/test_vg/lv01 using a block device level tool like iSCSI. A client then consumes the device and it appears inside the file system of the client as, say, /dev/sdc. On the client, an administrator now decides to also use LVM and sets up this device as a physical volume.

LVM on the client will now write a signature into /dev/sdc. This write will go through the iSCSI connection and the signature will be written to /dev/test_vg/lv01 on the server. If now LVM on the server scans the devices for signatures the next time, this signature will also appear on the server, and LVM will be confused and believe that a new physical device has been added.

To avoid this sort of issues, the LVM configuration file /etc/lvm/lvm.conf contains an option which allow us to add a filter to the scan, so that only devices which are matching that filter are scanned for PV signatures. We will need this when we later install Cinder which uses LVM to create logical volumes for virtual machines on the fly.

LVM snapshots

Let us now explore a very useful feature of LVM – efficiently creating COW (copy-on-write) snapshots.

The idea behind a copy-on-write snapshot is easily explained. Suppose you have a logical volume that contains, say, 100 extends. You now want to create a snapshot, i.e. a copy of that volume at a given point in time. The naive approach would be to go through all extents and to create an exact copy for each of them. This, however, has two major disadvantages – it is very time consuming and it requires a lot of additional disk space.

When using copy-on-write, you would proceed differently. First, you would create a list of all extents. Then, you would start to monitor write activities on the original volume. As soon as an extent is about to be changed, you would mark it as changed and create a copy of that extent to preserve its content. For those extents, however, that have not yet changed since the snapshot has been created, you would not create a copy, but refer to the original content when someone tries to read from the snapshot, similar to a file system link.

Thus when a read is done on the snapshot, you would first check your list whether the extent has been changed. If yes, the copied extent is used. If no, the read is redirected to the original extent. This procedure is very fast, as we do not have to copy around all the data at the time when the snapshot is created, and uses space efficiently, as the capacity needed for the snapshot does not depend on the total size of the original volume, but on the volume of change.

Let us try this out. For this exercise, we will use the logical volume /dev/test_vg/lv01 that we have created earlier. First, use fdisk to create a partition on this volume, then create a file system and a mount point and mount this volume under /mnt/lv/ . Note that – which confused me quite a bit when trying this – the device belonging to the partition will NOT show in in /dev/test_vg, but in /dev/mapper/, i.e. the path to the partition that you have to use with mkfs is /dev/mapper/test_vg-lv01p1. Then create a file in in the mounted directory.

(echo n; echo p; echo 1; echo ; echo ; echo w)\
  | sudo fdisk /dev/test_vg/lv01
sudo partprobe /dev/test_vg/lv01
sudo mkfs -t ext4 /dev/mapper/test_vg-lv01p1
sudo mkdir -p /mnt/lv
sudo mount /dev/mapper/test_vg-lv01p1 /mnt/lv
echo "1" |  sudo tee /mnt/lv/test
sudo sync

Note that we need one execution of partprobe to force the kernel to read the partition table on the logical device which will create the device node for the partition. We also sync the filesystem to make sure that the write goes through to the block device level.

Next, we will create a snapshot. This done using the lvcreate command as follows.

sudo lvcreate \
  --snapshot \
  --name snap01 \
  --size 128M \
  --permission r \
  test_vg/lv01

There are two things that should be noted here. First, we explicitly specify a size of the snapshot which is much smaller than the original volume. At a later point in time, when a lot of data has been written, we might have to extend the volume manually, or we can make use of LVMs auto-extension feature for snapshots (see the comments for the parameter snapshot_autoextend_threshold in /etc/lvm/lvm.conf and the man page of dmeventd which needs to be running to make this work for details). Second, we ask LVM to create a read-only snapshot – LVM can also create read-write snapshots, which in fact is the default, but we will not need this here.

If you now run lvs to get a list of all logical volumes, you will see that a new snapshot volume has been created which is linked (via the “origin” field) to the original volume. Let us now mount the snapshot as well, change the data in our test file and then verify that the file in the snapshot is unchanged.

sudo partprobe /dev/mapper/test_vg-snap01
sudo mkdir -p /mnt/snap
sudo mount /dev/mapper/test_vg-snap01p1 /mnt/snap
echo "2" |  sudo tee /mnt/lv/test
sudo cat /mnt/snap/test

In a real world scenario, we could now use the mounted snapshot as a backup, copy the files that we want to restore then eventually remove the snapshot volume again. Alternatively, we can restore the entire snapshot by merging it back into the original volume, which will reset the original volume to the state in which it was when the snapshot was taken. This is done using the command lvconvert.

sudo lvconvert \
  --mergesnapshot \
  test_vg/snap01

When you run this, the merge will be scheduled, but it will only be executed once the devices are re-activated. At this point, I got a bit into trouble. To understand the problem, let us first umount all mount points and then try to deactivate the original volume.

sudo umount /mnt/lv 
sudo umount /mnt/snap
sudo lvchange -an test_vg/lv01

But wait, there is a problem – when you simply run this command, you will get an error message informing you that the logical volume “is in use by another device”. It took me some time and this blog post describing a similar problem to figure out what goes wrong. To diagnose the problem, we can find the links to our device in the /sys filesystem. First, find the major and minor device number of the logical volume using dmsetup info – in my example, this gave me 253:0. Then, navigate to /sys/dev/block. Here, you will find a subdirectory for each major-minor device number representing the existing devices. Navigate into the one for the combination you just noted and check the holders subdirectory to see who is holding a reference to the device. You will find that the entry in /dev/mapper representing the partition that showed up after running partprobe causes the problem! So we can use

sudo dmsetup remove test_vg-lv01p1
sudo dmsetup remove test_vg-snap01p1

to remove these links for the original volume and the snapshot. Now you should be able to de-activate and activate the volume again.

sudo lvchange -an test_vg/lv01
sudo lvchange -ay test_vg/lv01

After a few seconds, the snapshot should disappear from /dev/mapper, and sudo lvs -a should now longer show the snapshot, indicating that the merge is complete. When you now mount the original volume again and check the test file

sudo partprobe /dev/mapper/test_vg-lv01
sudo mount /dev/mapper/test_vg-lv01p1 /mnt/lv
sudo cat /mnt/lv/test

you should see the original content (1) again.

Note that it is not possible to detach a snapshot from its origin (there is a switch –splitsnapshot for lvconvert, but this does only split of the changed extents, i.e. the COW part, and is primarily intended to be able to zero out those extents before returning them into the volume group pool by removing the snapshot). A snapshot will always require a reference to the original volume.

OpenStack Cinder foundations – storage networks, iSCSI, LUNs and all that

To understand Cinder, the block device component of OpenStack, you will need to be familiar with some terms that originate from the world of data center networks like SCSI, SAN, LUN and so forth. In this post, we will take a short look at these topics to be prepared for our upcoming installation and configuration of Cinder.

Storage networks

In the early days of computing, when persistent mass storage was introduced, storage devices where typically directly attached to a server, similar to the hard disk in your PC or laptop computer which is sitting in same enclosure as your motherboard and directly connected to it. In order to communicate with such a storage device, there would usually be some sort of controller on the motherboard which would use some low-level protocol to talk to a controller on the storage device.

A protocol to achieve this which is (still) very popular in the world of Intel PCs is the SATA protocol, but this is by far not the only one. In most enterprise storage solutions, another protocol called SCSI (small computer system interface) is still dominating, which was originally also used in the consumer market by companies like Apple. Let us quickly summarize some terms that are relevant when dealing with SCSI based devices.

First, every device on a SCSI bus has a SCSI ID. As a typical SCSI storage device may expose more than one disk, these disks are represented by logical unit numbers (LUNs). Generally speaking, every object that can receive a SCSI command is a logical unit (there are also logical units that do not represent actuals disks, but controllers). Each SCSI device can encompass more than one LUN. A SCSI device could, for instance, be a RAID array that exposes two logical disks, and each of these disks would then be addressable as a separate LUN.

When devices communicate over the SCSI bus, one of them acts as initiator and one of them acts as target. If, for instance, a host controller wants to read data from a SCSI hard disk, the host controller is the initiator, and the controller of the hard disk is the target. The initiator can send commands like “read a block” to the target, and the target will reply with data and / or a status code.

Now imagine a data center in which there is a large number of servers, each of which being equipped with a direct attached storage device.

The servers might be connected by a network, but each disk (or other storage device like tape or a removable media drive) is only connected to one server. This setup is simple, but has a couple of drawbacks. First, if there is some space available on a disk, it cannot easily be made available for other servers, so that the overall utilization is low. Second, topics like availability, redundancy, backups, proper cooling and so forth have to be done individually for each server. And, last but not least, physical maintenance can be difficult if the servers are distributed over several locations.

For those reasons, an alternative architecture has evolved over time, in which storage capacity is centralized. Instead of having one disk attached to each server, most of the storage capacity is moved into a central storage appliance. This appliance is then connected to each server via a (typically dedicated) network, hence the term SAN – storage attached network that describes this sort of architecture (often, each server would still have a small disk as a primary partition for the operating system and booting, but not even this is actually required).

Of course, the storage in such a scenario is typically not just an ordinary disk, but an entire array of disks, combined into a RAID array for better performance and redundancy, and often equipped with some additional capabilities like de-duplication, instant copy, a management interface and so forth.

Very often, storage networks are not based on Ethernet and IP, but on the FibreChannel network protocol stack. However, there is also a protocol called iSCSI which can be used to run SCSI on top of TCP/IP, so that a SAN can be leveraging existing IP-based networks and technologies – more on this in the next section.

Finally, there is a third possible architecture (which we do not discuss in detail in this post), which is becoming increasingly popular in the context of cloud and container platforms – distributed storage systems. With this approach, storage is still separated from the compute capacity and connected using a network, but instead of having a small number of large storage appliances that pool the available storage capacity, these solutions take a comparatively large number of smaller nodes, often commodity hardware, which distribute and replicate data to form a large, highly available virtual storage system. Examples for this type of solutions are the HDFS file system used by Hadoop, Ceph or GlusterFS.

The iSCSI protocol

Let us now take a closer look at the iSCSI protocol. This protocol, standardized in RFC 7143 (which is replacing earlier RFCs), is a transport protocol for SCSI which can be used to build storage networks utilizing SCSI capable devices based on an underlying IP network.

In an iSCSI setup, an iSCSI initiator talks to an iSCSI target using one or more TCP/IP connections. The combination of all active sessions between an initiator and a target is called a session, and is roughly equivalent to what is known as I_T nexus (initiator – target nexus) in the SCSI protocol. Each session is identified by a session ID, and each connection within a session has a connection ID. Logically, a session describes an ongoing communication between an initiator and a target, but the traffic can be spread across several TCP/IP connections to support redundancy and failover.

Both, the initiator and the target, are identified by a unique name. The RFC defines several ways to build iSCSI names. One approach is to use a combination of

The qualifier iqn to mark the name as an iSCSI qualified name
a (reversed) domain name which is supposed to be owned by whoever assigns the name so that the resulting name will be unique
a date (yyyy-mm) between the leading iqn and the domain name which is a date at which the domain ownership was valid (to be able to deal with changing domain name ownerships)
A colon followed by a postfix to make the name unique within the domain

As I own the domain leftasexercise.com since 2018, an example for a iSCSI name that I could use would be

iqn.2018-12.com.leftasexercise:foo

To establish a session, an initiator has to perform a login operation on the iSCSI target. During login, features are negotiated and authentication is performed. The standard allows for the use of Kerberos, CHAP and Secure remote password (SRP), but the only protocol that all implementations must support is CHAP (more on this below when we actually try this out). Once a login has completed, the session enters the full feature phase. A session can also be a discovery session in which only the functionality to discover valid target names is available to the initiator.

Note that the iSCSI protocol decouples the iSCSI node name from the network name. The node names that we have discussed above do typically not resolve to an IP address under which a target would be reachable. Instead, the network connection layer is modeled by the concept of a network portal. For a server, the network portal is the combination of an IP address and a port number (which defaults to 3260). On the client side, a network portal is simply the IP address. Thus there is an n-m relation between portals and nodes (targets and initiators).

Suppose, for example, that we are running a software (some sort of daemon) that can emulate one or more iSCSI targets (as we will do it below). Suppose further that this daemon is listening on two different IP addresses on the server on which it is running. Then, each IP address would be one portal. Our daemon could manage an unlimited number of targets, each of which in turn offers one or more LUNs to initiators. Depending on the configuration, each target could be reachable via each IP address, i.e. portal. So our setup would be as follows.

Portals can also be combined into portal groups, so that different connections within one session can be run across different portals in the same group.

Lab11: implementing iSCSI nodes on Linux

Of course, Linux is able to act as an iSCSI initiator or target, and there are several implementations for the required functionality available.

One tool which we will use in this lab is Open-iSCSI, which is an iSCSI initiator consisting of several kernel drivers and a user-space part. To run an iSCSI target, Linux also offers several options like the LIO iSCSI target or the Linux SCSI target framework TGT. As it is also used by Cinder, we will play with TGT today.

As usual, we will run our lab on virtual machines managed by Vagrant. To start the environment, enter the following commands from a terminal on your lab PC.

git clone https://github.com/christianb93/openstack-labs
cd openstack-labs/Lab11
vagrant up

This will bring up two virtual machines called client and server. Both machines will be connected to a virtual network, with the client IP address being 192.168.1.12 and the server IP address being 192.168.1.11. On the server, our Vagrantfile attaches an additional disk to the virtual SCSI controller of the VirtualBox instance, which is visible from the OS level as /dev/sdc. You can use lsscsi, lsblk -O or blockdev --report to get a list of the SCSI devices attached to both client and server.

Now let us start the configuration of TGT on the server. There are two ways to do this. We will use the tool tgtadm to submit our commands one by one. Alternatively, there is also tgt-admin which is a Perl script that translates a configuration (typically stored in /etc/tgt/target.conf) into calls to tgtadm which makes it easier to re-create a configuration at boot time.

The target daemon itself is started by systemctl at boot time and is both listening on port 3260 on all interfaces and on a Unix domain socket in /var/run/tgt/. This socket is called the control port and used by the tgtadm tool to talk to the daemon.

TGT is able to use different drivers to send and receive SCSI commands. In addition to iSCSI, the second protocol currently supported is iSER which is a transport protocol for SCSI using remote direct memory access (RDMA). So most tgtadm commands start with the switch –lld iscsi to select the iSCSI driver. Next, there is typically a switch that indicates the type of object that the command operates on, plus some operation like new, delete and so forth. To see this in action, let us first create a new target and then list all existing targets on the server.

vagrant ssh server
sudo tgtadm \
    --lld iscsi \
    --mode target \
    --op new \
    --tid 1 \
    --targetname iqn.2018-12.com.leftasexercise:tgt1
sudo tgtadm \
    --lld iscsi \
    --mode target \
    --op show

Here the target name is the iSCSI node name that our target will receive, and the target ID (tid) is the TGT internal ID under which the target will be managed. From the output of the last command, we see that the target is existing and ready, but there is no active session (I_T nexus) yet and there is only one LUN, which is a default LUN added by TGT automatically (in fact, the SCSI-3 standard mandates that there is always a LUN 0 and that (SCSI Architecture model, section 4.9.2), All SCSI devices shall accept LUN 0 as a valid address. For SCSI devices that support the hierarchical addressing model the LUN 0 shall be the logical unit that an application client addresses to determine information about the SCSI target device and the logical units contained within the SCSI target device.

Now let us create an actual logical unit and add it to our target. The tgt daemon is able to expose either an entire device as a SCSI LUN or a flat file. We will create two LUNs, to try out both alternatives. We start by adding our raw block device /dev/sdc to the newly created target.

sudo tgtadm \
    --lld iscsi \
    --mode logicalunit \
    --op new \
    --tid 1 \
    --lun 1 \
    --backing-store /dev/sdc

Next, we create a disk image with a size of 512 MB and add that disk image as LUN 2 to our target.

tgtimg \
  --op new \
  --device-type disk \
  --size=512 \
  --type=disk \
  --file=/home/vagrant/disk.img
sudo tgtadm \
    --lld iscsi \
    --mode logicalunit \
    --op new \
    --tid 1 \
    --lun 2 \
    --backing-store /home/vagrant/disk.img

Note that the backing store needs to be an absolute path name, otherwise the request will fail (which makes sense, as it needs to be evaluated by the target daemon).

When we now display our target once more, we see that two LUNs have been added, LUN 1 corresponding to /dev/sdc and LUN 2 corresponding to our flat file. To make this target usable for a client, however, one last step is missing – we need to populate the access control list (ACL) of the target, which determines which initiators are permitted to access the target. We can specify either an IP address range (CIDR range), an individual IP address or the keyword ALL. Alternatively, we could also allow access for a specific initiator, identified by its iSCSI name. Here we allow access from our private subnet.

sudo tgtadm \
    --lld iscsi \
    --mode target \
    --op bind \
    --tid 1 \
    -I 192.168.1.0/24

Now we are ready to connect a client to our iSCSI target. For that purpose, open a second terminal window and enter the following commands.

vagrant ssh client
sudo iscsiadm \
  -m discovery \
  -t sendtargets \
  -p 192.168.1.11:3260

Here we SSH into the client and use the Open-iSCSI command line client to run a discovery session against the portal 192.168.1.11:3260, asking the server to provide a list of all targets available on that portal. The output will be a list of all targets (only one in our case), i.e. in our case we expect

192.168.1.11:3260,1 iqn.2018-12.com.leftasexercise:tgt1

We see the portal (IP address and portal), the portal group, and the fully qualified name of the target. We can now login to this target, which will actually start a session and make our LUNs available on the client.

sudo iscsiadm \
  -m node \
  -T iqn.2018-12.com.leftasexercise:tgt1 \
  --login

Note that Open-iSCSI uses the term node differently from the iSCSI RFC to refer to the actual server, not to an initiator or target. Let us now print some details on the active sessions and our block devices.

sudo iscsiadm \
  -m session \
  -P 3
lsblk

We find that the login has created two new block devices on our client machine, /dev/sdc and /dev/sdd. These two devices correspond to the two LUNs that we export. We can now handle these devices as any other block device. To try this out, let us partition /dev/sdc, add a file system (BE CAREFUL – if you accidentally run this on your PC instead of in the virtual machine, you know what the consequences will be – loss of all data on one of your hard drives!), mount it, add some test file and unmount again.

sudo fdisk /dev/sdc
# Enter n, p and confirm the defaults, then type w to write partition table
sudo mkfs -t ext4 /dev/sdc1
sudo mkdir -p /mnt/scsi
sudo mount  /dev/sdc1 /mnt/scsi
echo "test" | sudo tee -a /mnt/scsi/test 
sudo  umount /mnt/scsi

Once this has been done, let us verify that the write operation did really go all the way to the server. We first close our session on the client again

sudo iscsiadm \
  -m node \
  -T iqn.2018-12.com.leftasexercise:tgt1 \
  --logout

and then mount our block device on the server and see what is contains. To make the OS on the server aware of the changed partition table, you will have to run fdisk on /dev/sd once and exit immediately again.

vagrant ssh server
sudo fdisk /dev/sdc
# hit p to print table and exit
sudo mkdir -p /mnt/scsi
sudo mount  /dev/sdc1 /mnt/scsi

You should now see the newly written file, demonstrating that we did really write to the disk attached to our server.

CHAP authentication with iSCSI

As mentioned above, the iSCSI standard mandates CHAP as the only authentication protocol that all implementations should understand. Let us now modify our setup and add authentication to our target. First, we need to create a user on the server. This is again done using tgtadm

sudo tgtadm \
    --lld iscsi \
    --mode account \
    --op new \
    --user christianb93 \
    --password secret

Once this user has been created, it can now be bound to our target. This operation is similar to binding an IP address or initiator to the ACL.

sudo tgtadm \
    --lld iscsi \
    --mode account \
    --op bind \
    --tid 1 \
    --user christianb93

If you now switch back to the client and try to login again, this will fail, as we did not yet provide any credentials. How do we tell Open-iSCSI to use credentials for this node?

It turns out that Open-iSCSI maintains a database of known nodes, which is stored as a hierarchy of flat files in /etc/iscsi/nodes. There is one file for each combination of portal and target, which also stores information on the authentication method required for a specific target. We could update these files manually, but we can also use the “update” functionality of iscsiadm to do this. For our target, we have to set three fields – the authentication method, the username and the password. Here are the commands to do this.

sudo iscsiadm \
  -m node \
  -T iqn.2018-12.com.leftasexercise:tgt1 \
  -p 192.168.1.11,3060 \
  --op=update \
  --name=node.session.auth.authmethod \
  --value=CHAP
sudo iscsiadm \
  -m node \
  -T iqn.2018-12.com.leftasexercise:tgt1 \
  -p 192.168.1.11,3060 \
  --op=update \
  --name=node.session.auth.username \
  --value=christianb93
sudo iscsiadm \
  -m node \
  -T iqn.2018-12.com.leftasexercise:tgt1 \
  -p 192.168.1.11,3060 \
  --op=update \
  --name=node.session.auth.password \
  --value=secret

If you repeat the login attempt now, the login should work again and the virtual block devices should again be visible.

There is much more that we could add – for instance, pass-through devices, removable media, virtual tapes, creating new portals and adding them to targets or using the iSNS naming protocol. However, this is not a series on storage technology, but a series on OpenStack. In the next post, we will therefore investigate another core technology used by OpenStack Cinder – the Linux logical volume manager LVM.