Can Someone explain me what really Open V means?
Thanks in Advance
Any server expects to see a drive geometry from a SCSI drive. The drive geometry is typically for open systems described in terms of Cylinders, Heads and Sectors. When you take eight drives and create for e.g a RAID5 group and then carve out LUNs from it to be presented to a server then you have to assign a emulation to the LUNs. The emulation is nothing but a CHS or Cylinder, Head Sector description. In the old HDS days you had fixed size emulations i.e OPEN-3 was 2.4 Gig, OPEN-8 was 7.3 Gig and so on. On the VSP the OPEN-V emulation can vary from 48MB to 4TB. My guess is that based on the size that you define the microcode decides what CHS to present to the server.
I don't know if this may be helpful but when teaching, I always say that "emulation defines how data is stored on the media and how it is accessed." Remember that Hitachi open systems storage "grew out of" mainframe storage systems. The Hitachi enterprise storage systems still support both mainframe and/or open systems emulations in the same storage system - but each RAID group supports only one or the other. The "how it is accessed" part of emulation for older emulations (OPEN-3, OPEN-9, OPEN-L, OPEN-M) included head of volume metadata which is handled in other ways for the OPEN-V emulation. While the metadata overhead space was not significant, OPEN-V doesn't need it. Again, "back in the day" when the older emulations were still supported and OPEN-V was "new" we said that OPEN-V always performs at least as well as any other emulation.
Emulation has always been a critical consideration in the Hitachi replication technologies since the P-Vol (source) and S-Vol (target) must be the same emulation.
All the newer, "cool" functionality uses only OPEN-V emulation - storage virtualization via thin provisioning (DP/HDT Pools) and external storage works only with internal LDEVs defined with OPEN-V emulation.
Again, emulation is still a storage attribute that must be specified when a RAID group is created and formatted into LDEVs but now the predominant emulation for open systems is OPEN-V.
The maximum OPEN-V LDEV size is actually now 60TB - but be a bit careful and be sure that all the functionality you need is supported for these very large LDEV volumes (I have not checked the replication large LDEV support lately....last time I knew with confidence the replication max was 4TB and this information may be dated)
My take is that a emulation is nothing but a way with which the storage array tricks the server into thinking that it is dealing with a SCSI drive with a particular geometry. All the server knows is the Target ID, LUN No and the returned values from the SCSI inquiry.
As far as the metadata requirement for the OPEN-3, OPEN-8 etc emulations is concerned I think was a limitation of the older RAID microcode whereby it needed a lookup table to translate CHS addresses to physical addresses on the underlying RAID group. The newer microcode seems to smart enough to use mathematics to convert CHS accesses for a OPEN-V emulation to physical RAID group addresses without a metadata requirement except in the case of HDP pools.
Emulations were the earliest form of Storage Virtualization both for mainframe and open systems.
In Hitachi enterprise storage systems, "emulation" is a "backend" attribute.
Consider this data transfer path
server <-> fabric <-> storage system channel host adapter port (CHA) <-> storage controller cache <-> disk adapter (DKA) or BED (backend director)<-> physical disk/RAID group.
Again, in Hitachi enterprise storage systems, "emulation" (like OPEN-V) is an attribute assignment that is set when the RAID group is created. Yes, the term "emulation" is used in many contexts in computer infrastructures, but for Hitachi enterprise storage, "emulation" is a storage system, "backend" attribute of the RAID group. when you want to change it, you must delete and recreate the RAID group - this is a disruptive operation and the data is lost from the disk media, due to reformatting.
Thus, in Hitachi enterprise storage systems, OPEN-V emulation defines "way that data is stored and accessed on the physical media."
It is helpful when you understand the Hitachi enterprise storage internal structure called the LDEV or logical device. When you want to learn more, consider one of the HDS Academy courses that cover enterprise storage.
There is nothing in the literature that distinguishes emulations based on the format in which data is stored and accessed on the physical media.
From what I read all emulations use the same Physical Drive format as well as the same RAID stripe size.
The only reference in the literature where there are differences between emulations is from the host attachment guides. See sample below
Thank you so much Vinod Subramaniam and Janet Hutchison
Thanks. I do hope my information and explanation was helpful....
Best of luck in your technology and storage adventures!
Thanks Janet. It was really helpful.
Retrieving data ...