Running Systems

Let’s say you have old shelves of either EMC or NetApp with SAS or SATA disks in them. And let’s say you want to connect them via FC to a Linux machine and have some nice ZFS machine/cluster, or whatever else. There are few things to know, and to attend in order for it to work.

The first one is the sector size. For NetApp – this applies only to non SATA disks (I don’t know about SSDs, though), and for EMC this could apply, as far as I noticed, to all disks – sector size is not 512 bytes, but 520 – the additional 8 bytes are used for block checksum. Linux does not handle well 520 blocks – the following error message will appear in the logs:

Unsupported sector size 520.

To solve it, we will need to identify the disks – using sg3_utils (in Centos-like – yum install sg3_utils) and then modify them to block size of 512 bytes. To identify the disks, run:

sg_scan -i
/dev/sg0: scsi0 channel=3 id=0 lun=0
HP P410i 3.66 [rmb=0 cmdq=1 pqual=0 pdev=0xc]
/dev/sg1: scsi0 channel=0 id=0 lun=0
HP LOGICAL VOLUME 3.66 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg2: scsi3 channel=0 id=0 lun=0 [em]
hp DVD A DS8A5LH 1HE3 [rmb=1 cmdq=0 pqual=0 pdev=0x5]
/dev/sg3: scsi1 channel=0 id=0 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg4: scsi1 channel=0 id=1 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg5: scsi1 channel=0 id=2 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg6: scsi1 channel=0 id=3 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg7: scsi1 channel=0 id=4 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg8: scsi1 channel=0 id=5 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg9: scsi1 channel=0 id=6 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg10: scsi1 channel=0 id=7 lun=0
SEAGATE SX3500071FC DA04 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg11: scsi1 channel=0 id=8 lun=0
FUJITSU MXW3300FE 0906 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg12: scsi1 channel=0 id=9 lun=0
FUJITSU MXW3300FE 0906 [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg13: scsi1 channel=0 id=10 lun=0
SEAGATE SX3300007FC D41B [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg14: scsi1 channel=0 id=11 lun=0
SEAGATE SX3300007FC D41B [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg15: scsi1 channel=0 id=12 lun=0
SEAGATE SX3300007FC D41B [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg16: scsi1 channel=0 id=13 lun=0
SEAGATE SX3300007FC D41B [rmb=0 cmdq=1 pqual=0 pdev=0x0]
/dev/sg17: scsi1 channel=0 id=14 lun=0
SEAGATE SX3300007FC D41B [rmb=0 cmdq=1 pqual=0 pdev=0x0]

So, for each sg device (member of our batch of disks) we need to modify the sector size.

Two ways to do so – the first suggested by this post here, is by using sg_format in the following manner:

sg_format –format –size=512 /dev/sg2

Another post suggested using a dedicated program called ‘setblocksize’. I followed this one, and it worked fine. I had to power cycle the disks before the Linux could use them.

I did notice that disk performance were not bright. I got about 45MB/s write, and about 65-70 MB/s read for large sequential operations, using something like:

dd bs=1M if=/dev/sdf of=/dev/null bs=1M count=10000
dd bs=1M if=/dev/null of=/dev/sdf oflag=direct count=10000 # WARNING – this writes on the disk. Do not use for disks with data!

Fairly disappointing. Also, using multipath, when the shelf is connected to one FC port, and then back to another, showed me that with the setting:

path_grouping_policy multibus

I got about 10MB/s less compared to using “failover” flag (the default for Centos 6). Whatever modification I did to the multipathd.conf, I was unable to exceed this number when using multiple access. These results were consistent when using multibus or group_by_serial, however, when a single path was active and the other was passive, It clearly showed better. I did modify rr_min_io and rr_min_io_rq, but with no effect.

The low disk performance could suggest I need to flush the original disk firmware, however, I am not sure I will do so. If anyone is reading this and had different results – I would love to hear about it.

While searching the web for how to perform PCI-Passthrough on XenServers, we mostly get info about previous versions. Since I have just completed setting up PCI-Passthrough on XenServer version 6. 5 (with recent update 8, just to give you some notion of the exact time frame), I am sharing it here.

Hardware: Cisco UCS blades, with fNIC. I wish to pass through two FC HBAs into a VM (it is going to act as a backup server, and I need it accessing the FC tape). While all my XenServers in this pool have four (4) FC HBAs, this particular XenServer node has six (6). I am intending the first four for SR communication and the remaining two for the PCI Passthrough process.

This is the output of ‘lspci | grep Fibre’:

0b:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)
0c:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)
0d:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)
0e:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)
0f:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)
10:00.0 Fibre Channel: Cisco Systems Inc VIC FCoE HBA (rev a2)

So, I want to pass through 0f:00.0 and 10:00.0. I had to add to /boot/extlinux.conf the following two entries after the word ‘splash’ and before the three dashes:

pciback.hide=(0f:00.0)(10:00.0) xen-pciback.hide=(0f:00.0)(10:00.0)

Initially, and contrary to the documentation, the parameter pciback.hide had no effect. As soon as the VM started, the command ‘multipath -l‘ would hang forever (or until hard reset to the host).

To apply the settings above, run (for a good measure. Don’t think we need it, but did not read anything about it): ‘extlinux -i /boot‘ and then reboot.

Now, when the host is back, we need to add the devices to the VM. Make sure that the VM is in ‘off’ state before doing that. Your command would look like this:

xe vm-param-set uuid=<VM UUID> other-config:pci=0/0000:0f:00.0,0/0000:10:00.0

The expression ‘0/0000’ is required. You can search for its purpose, however, in most cases, your value would look exactly like mine – ‘0/0000’

Since my VM is Windows, here it almost ends: Start the VM, and if it boots correctly, Install Cisco VIC into it, as if it were a physical host. You’re done.

I wanted to write down a guide for RHCS on RHEL/Centos6 and XenServer.

If you want to do that, you need to go through two major challenges which you will encounter. I want to save on the search and sum it all up together here.

The first difficulty is the shared disk. In order to set up most common cluster scenarios, you will need a shared storage. You could either map the VMs to an iSCSI LUNs external to the environment, however, if you do not have such infrastructure (either because everything is based on SAS/FC, or you do not have the ability to set up iSCSI storage with reasonable level of availability), you will want XenServer to allow you to share the VDI between two VMs.

In order to do so, you will need to add a flag to all your pool’s XenServers, and to create the VDI in a specific method. First – the flag – you need to create a file in /etc/xensource called “allow_multiple_vdi_attach”. Do not forget to add it to all your XenServers:

touch /etc/xensource/allow_multiple_vdi_attach

Next, you will need to create your VDI as “raw” type. This is an example. You need to change the SR UUID to the one you use:

xe vdi-create sm-config:type=raw sr-uuid=687a023b-0b20-5e5f-d1ef-3db777ce7ae4 name-label=”My Raw LVM VDI” virtual-size=8GiB type=user

You can find Citrix article about it here.

Following that, you can complete your cluster setup and configuration. I will not add details about it here, as this is not the focus of this article. However, when it comes to fencing, you will need a solution. The solution I used was a fencing agent which was written specifically for XenServer using XenAPI, by using the agent called fence-xenserver. I did not use the fencing agents repository (which this page also points to), because I was unable to compile the required components to run on Centos6. They just don’t compile well. This is, however, a simple Python script which actually works.

In order to make it work, I did the following:

• Extracted the archive (version 0.8)
• Placed fence_cxs* in /usr/sbin, and removed their ‘.py’ suffix
• Placed XenAPI.py as-is in /usr/sbin
• Verified /usr/sbin/fence_cxs* had execution permissions.

Now, I needed to add it to the cluster configuration. Since the agent cannot handle accessing a non-pool master, it had to be defined for each pool member (I cannot tell in advance which of them is going to have the pool master role when a failover should happen). So, this is my cluster.conf relevant parts:

<fencedevices>
<fencedevice agent=”fence_cxs_redhat” login=”root” name=”xenserver01″ passwd=”password” session_url=”https://xenserver01″/>
<fencedevice agent=”fence_cxs_redhat” login=”root” name=”xenserver02″ passwd=”password” session_url=”https://xenserver02″/>
<fencedevice agent=”fence_cxs_redhat” login=”root” name=”xenserver03″ passwd=”password” session_url=”https://xenserver03″/>
<fencedevice agent=”fence_cxs_redhat” login=”root” name=”xenserver04″ passwd=”password” session_url=”https://xenserver04″/>
</fencedevices>
<clusternodes>
<clusternode name=”clusternode1″ nodeid=”1″>
<fence>
<method name=”xenserver01″>
<device name=”xenserver01″ vm_name=”clusternode1″/>
</method>
<method name=”xenserver02″>
<device name=”xenserver02″ vm_name=”clusternode1″/>
</method>
<method name=”xenserver03″>
<device name=”xenserver03″ vm_name=”clusternode1″/>
</method>
<method name=”xenserver04″>
<device name=”xenserver04″ vm_name=”clusternode1″/>
</method>
</fence>
</clusternode>
<clusternode name=”clusternode2″ nodeid=”2″>
<fence>
<method name=”xenserver01″>
<device name=”xenserver01″ vm_name=”clusternode2″/>
</method>
<method name=”xenserver02″>
<device name=”xenserver02″ vm_name=”clusternode2″/>
</method>
<method name=”xenserver03″>
<device name=”xenserver03″ vm_name=”clusternode2″/>
</method>
<method name=”xenserver04″>
<device name=”xenserver04″ vm_name=”clusternode2″/>
</method>
</fence>
</clusternode>
</clusternodes>

Attached xenserver-fencing-cluster.xml for clarity (WordPress makes a mess out of that)

Note that I used four (4) entries, since my pool has four hosts. Also note the VM name (it is case sensitive), and your methods – one for each host, since you don’t want them running in parallel, but one at a time. Failover time is between 5-15 seconds on my tests, depending on who is the actually pool master (xenserver04 takes the longest, obviously). I did not test it with pool master down (before or without HA kicking in), nor with the hosts down and thus TCP timeout is longer (than when attempting to connect a host which responds immediately that it is not the pool master). However, if ILO fencing takes about 30-60 seconds, I am not complaining about the current timeouts.