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  • 1.  iscsi vm - performance less than expected

    Posted Jul 04, 2009 03:02 AM

    Hi All,

    Although not supported its always good to experiment. I have tried setting up a VM (tried various os's - solaris, ubuntu) as a iscsi target however when vm's sitting on this target write data performance is less than expected and I am seeing "read'" happening on the disk during the write process. Is this a VMFS alignment issue? All storage is happening local on a single disk.

    I have attached a picture to better describe what I am trying to do. (similar to a Sanmelody setup)

    Thanks in advance.



  • 2.  RE: iscsi vm - performance less than expected

    Posted Jul 04, 2009 07:26 AM

    By default all VMDKs are created in ZeroedThick mode. So you can try to inflate all VMDKs to EagerZeroedThick mode with "vmkfstools -k" command and then check speed again.

    zeroedthick (default) .

    Space required for the virtual disk is allocated during creation. Any data remaining on the physical device is not erased during creation, but will be zeroed out at a later time during virtual machine read and write operations.

    eagerzeroedthick

    Space required for the virtual disk is allocated at creation time. Unlike with the zeroedthick format, the data remaining on the physical device is zeroed out during creation. Disks in this format might take much longer to create than other types of disks.


    ---

    VMware vExpert '2009

    http://blog.vadmin.ru



  • 3.  RE: iscsi vm - performance less than expected

    Posted Jul 05, 2009 01:52 AM

    So if I write to the 2K3 VM Disk 2 - do you think its correct to see "reads" happening on the 500GIG physical disk?

    or would this setup with writing to the iSCSI create an extra write process?



  • 4.  RE: iscsi vm - performance less than expected

    Posted Jul 06, 2009 06:18 AM

    How much reads are happenning when you try to write?


    ---

    VMware vExpert '2009

    http://blog.vadmin.ru



  • 5.  RE: iscsi vm - performance less than expected

    Posted Jul 06, 2009 06:22 AM

    all that I can think of is I am now having "double" writes - write once, read, then write again for the same data



  • 6.  RE: iscsi vm - performance less than expected

    Posted Jul 06, 2009 07:17 AM

    You will get double writes in case of ZeroedThick disk. Write zeroes, then write data if it is first write attempt on block.


    ---

    VMware vExpert '2009

    http://blog.vadmin.ru



  • 7.  RE: iscsi vm - performance less than expected

    Posted Jul 06, 2009 10:42 AM

    Thanks for the reply's Anton.

    You do see that my iSCSI target's 200Gig disk is actually a VMDK sitting on the 500Gig physical.

    So we have 500gig VMFS formatted ---> 200GIG VMDK sitting on this ---> this vmdk disk is presented to a vm running solaris to make a iscsi target ->> presented to esx as iSCSI VMFS format -->> 2k3VM disk 2 20gig VMDKis sitting on this iscsi target.

    thanks again.