Confused with RAID5 Cage Availability

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I've got a new 2-node 7200 with 2 additional enclosures, 1 SFF, 1 LFF, so I have 2 SFF enclosures. Each one is half populated with 12 900GB SAS 10K drives.

I'm trying to setup a RAID5 CPG and it's telling me that there is not enough space on the system (this is the first CPG I'm trying to create). It will only let me setup the CPG if I change availablity from Cage to Magazine.

I thought Cage just meant it took chunklets from drives in both enclosures. Why can't I create any kind of RAID5 CPG using Cage availability with this setup? I've tried GUI and CLI. I can create a RAID1 CPG with Cage availability no problem.

[afidel]

Because cage availability is a rule that ensures you'll never lose data even if an entire cage (shelf in 7000) is offline. With only two shelves in your system there is no way for the system to ensure that. You need a minimum of 3 shelves (raid5 2+1) for RAID5 cage availability to work, and 4 shelves for RAID6 (6+2) to work, in the case of RAID6 you'll be in a vulnerable state if you use the minimum config but it's better than not having it.

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Of course, doh! I was visualising the chunklets across the disks left to right but for cage availability it's top to bottom through the enclosures. RAID5 over 2 enclosures would essentially be RAID10 with an unnecessary write penalty. I'll stick with the RAID1 then. Thanks.

[yizhar]

I've got a new 2-node 7200 with 2 additional enclosures, 1 SFF, 1 LFF, so I have 2 SFF enclosures. Each one is half populated with 12 900GB SAS 10K drives.

Hi.

Well, I'm a 3par newbie and just trying to figure out how to use our first 3PAR 7200 system (2-node SFF + LFF enclosure).

I would like to ask something about your hardware setup:

What's the point in having a half populated 7200 nodes SFF (12 drivers), and additional half populated SFF enclosure.
I would think of putting all SFF disks on the main 7200 2-node, and not use the additional SFF enclosure - just keep it for future use if/when you purchase additional SFF disks.
Splitting the disks between main and add-on encosure does not provide any better availability - if the main node enclosure is down - your whole system is also unavailable.
If the additional SFF enclosure is down - well, having all 24 SFF drives on the 2-node main enclosure would give better availability in that case.
Regarding performance - if I understand the 3par hardware architecture correctly then having a SFF full populated 2-node system will provide better (or at least the same) performance then 12 drives in main and 12 other in add-on enclosure.

Regarding power consumption, rack space, and the terrible noise from the enclosure, you can also be better without the additional enclosure.

So I would think that add a chained SFF enclosure only when the main is full and you want to add more SFF disks.

What do the other experts here suggest?

Yizhar Hurwitz,
Israel.

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I agree and I originally ordered 1 of each size enclosure but our partner chucked the extra SFF enclosure in for free stating HP best practice so I didn't argue and set it up this way so it would be easy to just expand the storage across two cages with additional disks in the future. I couldn't see any downsides to it but like you say, no real advantages when using half-populated enclosures either.