iSCSI rookies questions

Max Cuttins <max@xxxxxxxxxxxxx> · Tue, 12 Jun 2018 21:03:40 +0200



    Hi everybody,
    i have a running iSCSI-ceph environment that connect to XenServer
      7.2.

      I have some dubts and rookie questions about iSCSI.
    1) Xen refused to connect to iSCSI gateway since I didn't turn up
      multipath on Xen.

      To me it's ok. But Is it right say that multipath is much more
      than just a feature but it's a mandatory way to connect instead?

      Is this normal? I thought iSCSI multipath was back-compatible with
      singlepath one.

    
    2) The connection accomplished correctly with multipath.

      I see on the XEN dashboard:
    
      2 of 2 paths active (2 iSCSI sessions)

      
    I read around that for now that the iSCSI gateway would have just
      an active/passive multipath.

      Is this already worked? :)

    
    3) I see "optimized/not optmized" on my Ceph dashboard.

      This stand for?
    4) Performance.

      I run a simple test (nothing of statistically proven), and I see
      these value:

    
      dd if=/dev/zero of=/iscsi-test/testfile bs=1G count=1
        oflag=direct

        1073741824 bytes (1.1 GB) copied, 6.72009 s, 160 MB/s
      dd if=/dev/zero of=/ceph-test/testfile bs=1G count=1
        oflag=direct

        1073741824 bytes (1.1 GB) copied, 1.57821 s, 680 MB/s
    
    Of course I expected a drop (due to overhead of iSCSI)... but
      this is 4x slower than direct client. Which It seems to me a
      little bit high.

      However... is this more-or-less what I should consider as
      expected drop in iSCSI, or this gap'll be lowered in future?
    

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