I am trying to explain the terms used while configuring SAN. You can use the below details as reference for creating/reconfiguring LUNs in a SAN Storage.
The Model i worked on is EMC CX-320C. It had two DAEs (Disk Arrary Enclosures). DAE1 was populated with FC Drives and DAE2 was populated with SATAII. You must be able to figure out the options in other Storages easily.
EMC has an web interface for configuring the drives, RAID levels and to prepare them for storing data. It is called Navisphere Manager.
In the EMC's interface you will see a list of things - Hosts connected to the SAN, physical interfaces, SPA, SPB details ( drives in that ), RAID Groups and Storage Groups. Initially there wont be any RAID/Storage Group.
First we have to configure the drives into specific RAID (0/2/5) groups and then add them to a Storage group.
RAID Group: (RG)
A collection of drives operating in a RAID. When creating a new RAID group you have to specify the following:
1. RAID group id (Unique).
2. Number of disks you are going to add (you can select the drives from specific DAE).
3. RAID Type - HotSpare, Disk, Unbound (more on this later). If selecting more than 3 drives RAID 0,3,5 will show up.
Best practices while creating RGs:
1. Do not combine the different disk types into single RG. Combining them will make the RAID array to operate in lowest speed available.eg: Create separate RGs for FC drives and SATAII drives. Combining them will make FC drives to operate in SATAII's speed.
2. Remember to have one drive as global hot spare for the whole array/DAE so that it will kick in when one of the drives in the array fails.
3. RAID0 is for highest performance (no redundancy) while RAID5 is for high redundancy (ofcourse has performance hit).
4. Do not combine drives from 2 DAEs to a single RG even if they are of same type and speed.
LUN:
After Creating a RAID group you will create LUNs. LUNs are nothing but partitions presented as (scsi) drives to servers. LUNs are always a part of the RAID group. You cannot assign a RG as whole to a Storage Group, instead you will assign individual LUN/s to SG/s.
If you have specified 'unbound' as RAID type while creating RG, you can specify the RAID type when creating a LUN. Remember all LUNs in a RG must of same RAID Type.
Storage Group:
Storage Group is a collection of all info: LUNs, Hosts, SAN Copy connections etc., (we have all these three and there may be more that i am not aware of). A unique name must be given for every storage group.
Create a Storage group as you desire and add LUNs and hosts to it. You can either select LUNs & connect hosts from the Storage group or add LUN to a Storage Group from a RG.
Note the following:
1. You can add one LUN to multiple storage groups. But remember that only one server from the SGs must access the LUN. If you want more than one server to access a single LUN simultaneously then that LUN must be configured as a file system cluster (eg: gfs).
2. You can add a host to only one SG. Adding the same host to another SG will remove the host from the old SG.
3. There can be as many LUNs in a SG.
