Create Centralized Secure Storage using iSCSI Target / Initiator on RHEL/CentOS 7 – Part 12

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Gabriel Cánepa

Gabriel Cánepa is a GNU/Linux sysadmin and web developer from Villa Mercedes, San Luis, Argentina. He works for a worldwide leading consumer product company and takes great pleasure in using FOSS tools to increase productivity in all areas of his daily work.

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4 Responses

  1. Vasanth KS says:

    Great article. But everything was fine till.

    # iscsiadm -m node -T -p -l

    The above command throwing an error and not attaching.

    Logging in to [iface: default, target:, portal:,3260] (multiple)
    iscsiadm: Could not login to [iface: default, target:, portal:,3260].
    iscsiadm: initiator reported error (24 - iSCSI login failed due to authorization failure)
    iscsiadm: Could not log into all portals

    Please help me..

  2. Ek C. says:

    Hello Gabriel,

    Many thanks for the article.

    The step-by-step approach is easy to follow. I like it.

    One comment, the mount option in /etc/fstab should be _netdev instead of default to wait for network before a mount attempt.

  3. Thomas Williams says:

    That was a good article – thanks Gabriel. I would add a couple things. The iSCSI protocol is very robust, but the applications running on the server have an expectation of high reliability and low latency. After all, iSCSI is emulating a local hard disk, and if the disk were to start having long latency or suddenly disappear, the running applications will likely crash. Having said that, your best chance of success happens when you use an isolated, dedicated network for the iSCSI SAN. That means a dedicated NIC on all of the servers, and a dedicated Ethernet switch for the network. There should be no other traffic on the SAN, and no access to the Internet or the enterprise networks via the SAN. All of the devices on the SAN should be running at the same speed (1Gb, 10Gb, etc.). Do not mix different speed devices on the SAN. The iSCSI will work with 1500 Byte MTU, but you will see better performance with a 9000 Byte MTU on all attached devices. A common problem with iSCSI SAN’s is “out-of-order” frames (it is not supposed to, but yes, it happens). When frames arrive out of order, the receiving NIC must buffer the frames until the missing frame arrives. If the missing frame does not arrive before the buffer overflows, frames are dropped, the sender waits for the timeout period, and then re-transmits all of those frames. To check for re-transmits do “netstat-s” on the sending side.
    To minimize the re-transmit problem, use ethtool to increase the receive buffers on the SAN NICs. The details vary depending on the NIC and driver, but generally it looks like this:


    ETHTOOL_OPTS=”-G rx 2047 rx-jumbo 1023″

    The idea here is to increase the receive buffers to the maximum possible size on the SAN NICs.
    Last, you want to use an “Enterprise” class Ethernet switch, meaning non-blocking on all ports and large packet buffers. One example is Dell PowerConnect 4000 series. There are many others that work too, but avoid those small, cheap, office grade switches.

    The iSCSI SAN is a viable alternative to Fiber-Channel when done correctly. It also has a much smaller price tag. I hope this helps.


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