iSCSI target shared by multiple machines?

Posted on 2012-08-30
Last Modified: 2012-08-30
I have a Seagate Blackarmor 440 2 TB drive that I've set up a 1.5 TB iSCSI drive on.

I currently have it mapped on my workstation for media files (archived) and have just added it to our sql server 2005 on server2008 VM as a target for daily backups.

I've not used iSCSI before so I am surprised to be told by "my guy" that it should only have 1 client pointed at it, especially after reading the seagate docs which mention nothing of the sort.

Seems a bit odd (and inconvenient and not particularly useful) that I would have to set up a drive for each client.

Can someone clarify proper usage.
Question by:SidFishes
    LVL 1

    Assisted Solution

    It's best practice to have only one machine connected to an iSCSI Target at a time. (You don't want two machines to be able to change the data one the same volume...causes issues when both are trying to change the same data)

    That being said, this doesn't mean that you can't set up multiple iSCSI targets on a single iSCSI device and direct the differnet machines to their own iSCSI target.

    If both are reading / writing to the iSCSI storage at the same time, obviuosly performance will be affected.
    LVL 1

    Assisted Solution

    depends on your purposes but you can definitely have more than 1 client pointed at it.  Think of VMware implementations pointing to a SAN, multiple hosts using iSCSI to access the same target.  you just want to make sure your hosts are not accessing the same DATA on the target at the same time.
    LVL 36

    Author Comment

    so the answer is no & yes :)

    in practice, my backups run at 1 am, i never touch those files (except for disaster recovery) and the sql server never touches the files outside of the backup directory.

    I don't mind setting up a second target just for the backups but i'm surprised that this seems to be so fragile
    LVL 2

    Assisted Solution

    If the file system know that it is on a shared iSCSI like the vmfs 5, then everything is fine. If you use a NTFS than you may have errors and problems. It all comes down to file locking.
    LVL 55

    Accepted Solution

    Without clustering software such as aforementioned VMware, MSCS, GFS etc corruption is inevitable if two machines access the same LUN. With a normal filesystem such as FAT, NTFS, EXT3 etc the file allocation table / inode table or whatever you want to call the index is stored in the computer's RAM to save having to access disk for that and only flushed occasionally. When you do a write with a non-clustered filesystem the data is put on the disk and the index is stored in RAM, then the next computer comes along and writes straight ontop because it isn't aware that the other computer wrote anything there. Try it out for yourself on a test LUN, write a file from one machine to a shared LUN and then get a directory listing from another - the file isn't seen to be there.

    Multiple targets (LUNs) as above or normal network fileserver shares don't suffer this problem so just use the NAS feature of your box rather than iSCSI.
    LVL 36

    Author Comment

    "Try it out for yourself on a test LUN, write a file from one machine to a shared LUN and then get a directory listing from another - the file isn't seen to be there."

    actually seeing this - the first backups from yesterday can't be seen from my workstation (which was the reason for me talking to "my guy")

    Creating a dedicated 500 iSCSI target for the backups. (the drive is 3 TB not 2)

    Thanks all.

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