Hello Scooby,
1) Correct, a disk can be (and usually is) in several CPGs. In my case, I utilize 4 types of CPGs. First is to differentiate between SATA and FC, and that is a hard separation of spindles... I have no CPGs that span both FC and SATA (I also don't have subvolume tiering)... The other 2 types I employ dictate raid type, and inner/outer track preference. I use raid5 with cage availability for production CPGs using the "fast" outer tracks (in my case thats 5+1), but for dev CPGs I use magazine availability (raid5 8+1) and the "slow" inner tracks. As you can see, my DEV and Prod share the same spindles, just one prefers the slow tracks, and the other prefers the fast ones. You can also use it to segregate data as well, such as if you're following a DB best practice to the letter... have data on even numbered magazines and logs on odd numbered ones, but for all other data use all magazines.
2) Thats true too, although you can set limits if you like/need per CPG. Multiple CPGs can pull free space from the drives. First come first serve, this could get tricky when you have a special CPG for say database logs that can only grow on certain disks, and you have other CPGS that can grow anywhere. Monitoring freespace on a PD level, or using a CPG strategy that does not limit PDs may be desirable. If a special CPG limited to portion of the systems starts to get full, I believe you can edit it to be less restrictive, and it will use the new disks for growth as well. At that both the majority of the data will be on the original set of disks, and you could use Dynamic Optimizer to rebalance that if you like.
I'm not sure I am completely used to the CPG concept yet either. I started on Inserv 2.2.4 and back then the CPGs were limited to 64 VVs, but they recommended only using 32. I was told that with 2.3.1 those limitations have been changed/removed/fixed but I've been sticking with 32 per until I learn more about it.
I believe that 3PAR standard is to steer customers to leaving the CPGs wide open with access to all the disks of the same type (FC vs SATA, unless you have Sub-Volume Tiering which I don't) and let the Inserv's wide striping do its thing. They can separate physical disk by usage, such as commonly a best practice for Databases, and I have never read anything from 3PAR that refutes that strategy, probably to play nice. Perhaps on smaller systems with a low spindle count, this might be necessary... However, my personal experience with several hundred spindles is that it is not needed and wide striping works as advertised. By all means, have this conversation with your 3PAR/HP technical sales engineer, or support to get specific recommendations tailored to your environment.
_________________ Richard Siemers The views and opinions expressed are my own and do not necessarily reflect those of my employer.
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