How I fixed an iSCSI link and found 28 bytes
anthonyv 2000004B9K Comments (2) Visits (5191)
One of the many popular features of the XIV is the ability to replicate using iSCSI. On XIV Gen3 there are now at least 10 and up to 22 active iSCSI ports on each machine (depending on how many modules you order).
Implementation of the iSCSI connection between two XIVs is a piece of cake. If both XIVs are defined to the XIV GUI (which they should be), you just need to drag and drop links between XIVs to bring the iSCSI mirroring connections alive. If the network gods are with you, the link goes green. But... if the networking gods are against you... the links staysred and then the question is... what to do?
Old fashioned problem diagnosis leads us straight to the ping command. However I routinely find that the ping command works fine (all interfaces respond), but the link stubbornly remains red.
The first possible problem is that iSCSI uses TCP port 3260, so hopefully there are no firewalls blocking that port.
The second possible problem is the MTU size (Maximum Transmission Unit). When we define the iSCSI interfaces on the XIV we set the MTU as a value of up to 4500 bytes. When we establish connections between two XIVs, each XIV will send test packets that are sized to the MTU. If the intervening network does not support that packet size, the packets will be dropped by the network, because the XIV sets the don't fragment flag to ON.
So how to work out what the MTU is? Well the first thing to do is ask your friendly networking team member. But sometimes I find that the intervening networks are controlled by third parties, which means that getting a straight (and reliable) answer can prove difficult. Even worse, some of these third parties charge a fee every time you call them, so there may be hesitation to even get them involved!
One simple trick is to re-use the ping command but play with payload sizes. We can use a command that looks like this:
ping -f -l 1472 192.168.0.1
That command sends a ping with a payload of 1472 bytes to IP address 192.168.0.1. We add the -f parameter to prevent packet fragmentation. What you then do is slowly increase the payload until you no longer get a reply.
This process works fine and is great way to determine the maximum payload size the end to end network will support. However if you're using the payload size to determine the maximum transmission unit, there is a little trick. The MTU is the maximum packet size, but a ping sends a payload wrapped in 28 bytes of IP and ICMP headers. So our example:
ping -f -l 1472 192.168.0.1
sends a 1500 byte frame to the 192.168.0.1 IP address (1472 bytes of payload and 28 bytes of headers.
If this command succeeds, you can use an MTU of 1500 in the XIV GUI or XCLI (rather than an MTU of 1472, which is 28 bytes smaller).
For those who are wondering how I did the networking sniffing to get the screen captures above, I used a brilliant piece of freeware software called Wireshark. My only warning is that your corporate security policies may have rules on sniffing the network. Don't take my blog post as permission to use it # And for the networking geeks among you, yes I know that extra packets could actually be wrapped around our ethernet packet for things like VLAN tags or encapsulation, but hopefully this should not affect our mathematics.
Controlling the background traffic
Final pointer. Having finally gotten the link up and going, you are now free to start replicating volumes. But how much traffic can the cross site link support? The XIV can limit the background copy bandwidth with a parameter called max_
For each target you should see three parameters:
These three settings should be tuned to reflect the possible throughput of the cross site links.
To change the settings use a command like this (change the target name and the rates to suit):
If you want to see the current throughput rate, open XIV Top on the remote machine. You should see how much write I/O is being sent to the mirror target volumes in MBps.
So hopefully your now better positioned to diagnose iSCSI link issues, maximize your MTU and tune and monitor your link speed.
Questions? Fire away...