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EVA LUN numbers and Vdisks

mohannad
Frequent Advisor

EVA LUN numbers and Vdisks

i have 2 questions regarding the EVA command view and VDISKS:

1- what is the diffrence bettwen the 2 buttons in the vdisk presentation " Assign LUN " and " Present Vdisk" in the command view the for what i can make they both do the same thing but assign lun gives you the option of selecting the LUN number you want?

2- why do we have a lun number if each vdisk has a uniqe WWN why dose it need a lun number, lun numbers was used to address the lun in relevance to the array uniqe WWN so if EVA assignes a uniqe wwn for the lun this is enough to address the lun at hand why would we need a lun number also ?


10 REPLIES
Mustafa Gulercan
Respected Contributor

Re: EVA LUN numbers and Vdisks

hi;
1-if you use"Present Disk" button EVA will give the next LUN ID to present it.
But if you use "Assign LUN" button you will assign it manually.

2-EVA use the LUN ID for hosts.
because when you ioscan from the host, you will see the vdisk like cxtxdx.the number of txdx is calculated from this LUN ID.

regards,
mustafa
Torsten.
Acclaimed Contributor

Re: EVA LUN numbers and Vdisks

There are still operating systems that count on LUNs.

Hope this helps!
Regards
Torsten.

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mohannad
Frequent Advisor

Re: EVA LUN numbers and Vdisks

thanks guys,

so mustafa what your saying both buttons dose the same thing but i can use the assign lun to assign a lun ID.

but regardin the lun number im not convinced, bare with me guys lets say there is still some OS's using the lun number there would be a confilct if we assigned 2 difrent luns from 2 diffrent storages with the same lun lets say 1 and 1 ??????????



Torsten.
Acclaimed Contributor

Re: EVA LUN numbers and Vdisks

It depends on the OS.

For example HP-UX will know that thess LUNs are from different arrays (based on hardware path).

The OS will know this, but it could be hard for the admin ... better don't do this.

Hope this helps!
Regards
Torsten.

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mohannad
Frequent Advisor

Re: EVA LUN numbers and Vdisks

so what your saying the tradditional or old way was lun numbers but noe its the UUID ? is that correct
Torsten.
Acclaimed Contributor

Re: EVA LUN numbers and Vdisks

Well, it depends ...

What is your OS?

Hope this helps!
Regards
Torsten.

__________________________________________________
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Uwe Zessin
Honored Contributor

Re: EVA LUN numbers and Vdisks

All operating systems talk through the SCSI LUN 'number' to a virtual disk, because this is the only addressable entity. A 64-bit WWN is a characteristic of a Fibre Channel port as a 128-bit LUN-WWN is a characteristic of a virtual disk's LUN mapping, but they cannot be used as direct addresses.

> would be a confilct if we assigned 2 difrent luns from 2 diffrent storages with the same lun

No conflict, because there are other entities (controller, bus/channel, target) which are used to form a full address.

Different ports from the same or another storage array are simply mapped to different SCSI target addresses.
.
mohannad
Frequent Advisor

Re: EVA LUN numbers and Vdisks

well torsten its not a specific OS type i was just wondering why there is 2 addresses for the LUN, the LUN ID and the LUN WWN it dosent make sense to hav 2 adresses.

and yes its true the N_PORT ID is 64 bit addresses, and 128 bit are LUN addresses. i am talking about the 128 LUN addresses - the UUID of a vdisk - , so what are you saying Uwe this is assigned for a vdisk by the eva but its not used for host to LUN addressing ?

actually i read a piece of info somewhere im not sure of it that the UUID is assigned by the storage system for internal addressing acctually if i remember correctly it stated the storage system assignes a 128 bit Uniqe identifier for each object in the storage for internal addressing is that correct and is that what the vdisk UUID used for ?
Uwe Zessin
Honored Contributor

Re: EVA LUN numbers and Vdisks

The N_Port ID (Fibre Channel address) is 24 bits and is something different than a 64-/128-bit WWN. It is used to access physical ports in a Fibre Channel fabric and works (a bit) like an IP-address while a 64-bit WWN works (a bit) like a MAC address. A SCSI target address is mapped to an N_Port ID by the operating system device driver.

> the UUID of a vdisk ... its not used for host to LUN addressing ?

No, not the UUID, and not the LUN WWN. A UUID is used to persistently identify an object (disk drive, controller, host, vdisk, ...) _within_ an EVA and does not change. By default, the LUN WWN has the same value as the UUID, but you can be changed it if the VDisk is not presented to a host.
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mohannad
Frequent Advisor

Re: EVA LUN numbers and Vdisks

ok Uwe, thanks for the explination but im more confused now, you hit the spot here, i have been working with storage for about a year now EMC and HP and yet i cant find any refrencing meterial from both to the SAN infrastructure that explains the SAN as a network and the infrastructure involved in it , do you have any reading material for this subject or any idea where could i find something like that?