Help - setting up iSCSI node

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  • Last Post 13 October 2017
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Luca Baffa posted this 05 October 2017

Hello everybody!

I have no prior experience with iSCSI and I would like to ask for a little guidance.

I currently have a VessRAID 1830i up and running. There is a link aggregation between all the four ports of the controller and a windows server (initiator) mounts at boot the folders of the VessRAID. It all works (with my great happiness).

 

Now, I need to set up a new Debian server which should mount the VessRAID partitions (via iSCSI) without interrupt the service on the windows server.

I was hoping to initiate a new iSCSI session on the Debian server, and access the files both from the windows and the Debian server. But I soon found out that it's not possible to have multiple connections on the same ports.

So, can you confirm me that what i need to do is to modify the present linked aggregation from:

(port1 master, port2 slave, port 3 slave, port4 slave)

to two aggregations: (port1 master, port2 slave) (port 3 master, port4 slave)

Then create a new iSCSI node on the new link and after that I would be able to initiate the connection with open-iscsi on Debian?

Would the windows server be influenced by it (beside, i assume, a very short network disconnection and a lower transferring speed)?

Is there any precaution you would advise me to take before I start?

 

Thanks very much!

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R P posted this 05 October 2017

Hi Luca,

From your post, it looks like you intend to connect 2 servers to the same iSCSI LUN. This is a really bad thing to do unless you have a clustered filesystem, and if you are using Windows now, NTFS is not a clustered filesystem. While it may seem to work at first, there is a danger that your filesystem will be irrecoverably lost. Do not connect more than 1 computer to a LUN unless it has a clustered filesystem such as VMware's VMfs.

You can connect multiple initiators to the same iSCSI LUN on the same portal, in forum thread you linked to I connected 8 initiators to the same iSCSI LUN at the same iSCSI portal (with IP 10.81.49.25).

administrator@cli> iscsi -a list -t session
===============================================================================
Id    CtrlIdTargetAlias    InitiatorAlias PortalId PortalIP         Status
===============================================================================
1     1     alias          vmware-1       0     10.81.49.25         Active
2     1     alias          vmware-1       1     10.81.49.26         Active
3     1     alias                                0     10.81.49.25         Active
4     1     alias                                0     10.81.49.25         Active
5     1     alias                                0     10.81.49.25         Active
6     1     alias                                0     10.81.49.25         Active
7     1     alias                                0     10.81.49.25         Active
8     1     alias                                0     10.81.49.25         Active
9     1     alias                                0     10.81.49.25         Active

Note, this was done to test the connection limit, I did not create a filesystem on the LUN or attempt to write to it or read from it.

You can connect multiple computers to different LUNs at the same portal. But if you have 2 identical sized LUNs it can be difficulty to see which LUN is used and connecting multiple computers to the same LUN is possible. The risk of this can be mitigated by LUNmapping each LUN to the appropriate computer's initiator.

If you do need to share files between multiple computers, then the best solution would be to create a NAS volume on the Vess. But you need to be aware that Linux and Windows have different limitations on what is acceptable in filenames, it is not advisable to use any special characters on filenames when files to be shared between different operating systems, even if one operating system allows them. It's best to stick to uppercase and lowercase letters, numbers, and the space ( ), dash (-) and underscore (_).

Another option is to reshare the volume through Windows or Linux.

Regards,

--Rich

 

 

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Luca Baffa posted this 06 October 2017

Thanks very much Rich!!

Ok! I clearly underestimate the complexity of the iSCSI connection.

I think the option that suits best my needs is to 'unlink' some LUNs from the windows initiator and 'assign' them to the linux server. Eventually I'll get rid of the windows server, but for the moment it's important to do one thing at time.

Could you suggest me some readings that might help me bridge my knowledge gap?
I found a nice one on the micrisoft website, but I still miss some pieces of the puzzle (for example, what this LUNs really are).

Thanks very much for your time and patience.

Luca

R P posted this 06 October 2017

Hi Luca,

Thanks for the additional detail, you have multiple LUNs. I was afraid that you had 1 big LUN (very common) and intended to connect both the Linux server and Windows server to it, which, as I said, is a very bad idea. It's good to see that this is not the case.

A LUN is a Logical Drive (LUN stands for Logical Unit Number), as opposed to a physical drive. A physical drive is fixed an cannot be changed, but a LUN is typically created from multiple drives, so you can create a LUN of arbitrary sizes to meet your needs. In the Vess you would create an array (in this context it's just a collection of drives logically bonded together) and create one or more LUNs on it. If you had 12 1TB drives you would have 12TB of raw capacity, and if you used all the space you could create an 11TB RAID5 (1 drive's capacity is used for parity data), or a 8TB R5 and a 3TB R5, or whatever would meet your needs.

iSCSI is simply SCSI over ethernet. The details are not important, but when you connect to an iSCSI LUN with your initiator you will see a raw disk, and Windows will ask if you want to initialize and format it, so you'd put an NTFS filesystem on it and you will see a disk in Windows the size of your LUN connected through ethernet.

From the sound of it you seem to be able to work with the Linux and Windows iSCSI initiators, so you seem to be in good shape in that regard.

Regards,

--Rich

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Luca Baffa posted this 13 October 2017

Hi Richard!

Sorry for the late reply!

Thanks so much for all of your help! I'll run some tests and modify the Windows iSCSI initiator as we discussed.

Thanks very much!

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