I am new to Hitachi and want to know what is LU Path & Control Unit and what are the use cases of them can anyone explain me?
My answer will not be comlpete but here are a few things that pop to mind:
1. Referred to as CU:LDEV - or Control Unit:Logical Device, example : 40:10. Also abbreviated as just LDEV as in "Which LDEV is that?" "oh it is 4010". maybe that is Hitachi-lingo slang?
2. Used for identifying LUNs on the array. It is two Hex digits (00-FF) for both each of CU and LDEV. FF hex = 256 decimal, 256 CUs and 256 LDEVS = 256 x 256 which is no coincidence equal to the max number of LUNs on a hitachi array (USPV, VSP anyway) of 64,000 (might be 65536 or something closer to that).
3. Where to use? First off all LUNs that are created out of disk space on the array need to be identified some way, and that way is a CU LDEV. You can also have frendly names in VSP but the system uses the CU:LDEV.
In pooling an example is you create CU:LDEVs on the disk Array Groups, these are LUNs - then you put this LUNs into a pool as the feed stock to make up the capacity of the pool (called Pool-VOLs), then you create Virtual Volumes (VVOLs, or DPVOLs) from that thin provisioned wide striped HDP pool and you also identify those with CU:LDEV assignments too.
Here is where the crazy starts. We use certain CU:LDEV ranges for carving the array up initially (for example 40:00 44:FF), FE:-- - FF:FF reserved for specific to the storage group use (command devices, journal devices, system disk etc). Back when we were very rigerous we had a table of CU:LDEV ranges for each of 1, 4, 16, 32, 64, 128, 256, 512 GB LUNs so if you saw a LUN 73:23 you knew it was a 64GB LUN for instance (I am just making these up these translations here for illustration purposes). We kept 00:00 to 3F:FF unused on USPV and VSP in case we ever migrated a USP (USP topped out at 3F:FF) to a newer product then those LUN ID's would not already be in use and we could move them over without change (a good idea on paper, we never ended up using it).
We have other uses for CU LDEV too but hopefully you get the point.
HCS software likes to start at "00:00" and go from there, which is frustrating if you make a scheme like we did. There are ways around the restrictions and it does not happen everywhere and over time the restriction is getting less.
You can use CU:LDEV however you want: Markeing gets CU starting at 60:xx, Engineering is 9A:xx, and Shipping is B0:xx and above.
Warm welcome to Hitachi world!
CU stands for Control unit which is a logical entity. All the logical devices(LDEVS) that have been carved out from a RAID group have to be part of a Control Unit.
Parity Groups are created from physical disk.
RAID level and emulation is applied to the group.
LDEVs are mapped as LDKC#:CU#:LDEV
An LDKC is a set of control tables. LDKC can be set to 0 and 1.
LU path: LUN path which is mapping of LUN to host. Require below information to set LU path:
Port: The name of the port to which the host group belongs.
HostGrpID :The host group number linked to the logical volume.
LUN: Indicates the LUN of the logical volume linked to the host group.
LDKC:CU:LDEV The LDKC, CU, and LDEV numbers of the logical volume linked to
the host group.
Num. of Paths The number of LU paths set.
Hope it helps.
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