RAID groups for backup to disk on CX4-240

Welcome to the forums, Moresukine !

I think you prepared yourself well

What I did in the past to determine the optimal RG size is a bit sneaky: I looked at the config of an old CDL (Clariion Disk Library) and copy the RG setup from there (4+1 and 8+1), but nowadays with the very large disk sizes as you mention the old CDL setup is not be the optimal one anymore, since Raid6 is probably the better and safer choice.

Purely based on numbers I'd say 3 x (10+2) plus 2 HS's is a good choice, since you have no drives left over and the number of HS's is about right too, but I know there's a optimal stripe size which involves stripes to either 4 or 8 drives, so in your case the (8+2) is the cheapest (cheaper than (4+2) or something), but still safest way (Raid6), but that'll leave you with 3 x (8+2) plus 6 HS's, which is a bit overkill on the HS side.

Could you go to 4 x (8+2) plus 2 HS's ? You still need 3 DAE's, which you already have, so there's no extra cost there, just 4 extra drives.

I know this is only half the story, so I'd like to know if this assumption of mine that you should choose 8+2 over 10+2 is valid. Anyone ? I can explain why smaller RG's for random I/O are better, but I always seem to get stuck in translation with sequential data streams....

The perferred configuration is the 8+2.  This will generally have better caching performance than the 10+2.  BTW, this is the configuration that is used by EMC's backup suite.

Mike,

You said that the CX4-240 has three DAEs populated with SATA drives. Are one of those DAEs the DAE-OS? It will be bus 0 encl 0. If so then I would not use disks 0-4 as part of a RAID group used as a backup to disk target.

Aran

I thought so, but I had no proof to support my story

So the new DL's also have Raid6 in 8+2 ? Do you know if all raid groups are 8+2 or is it a mix like on the old CDL's (4+1 and 8+1) ?

I don't know.

The beauty of the 8+2 is its 512KB stripe size.  It aligns real well with a backups I/O size.

Aran, i think that the only CX4 array that allows SATA II in  enclosure 0-0 is a CX4-120.

AranH was a bit confused about SATA drives in the CX4-240's first enclosure. Forgive him. I think he nees a vacation..... 

I do and I was ...

I couldn't remember which models could use SATA for the vault disks. Those models are just so small, I never work with them.

The CX3-10 can use SATAN drives as vault disks. All other models can only have FC in the first enclosure.

> I will contact our supplier and come back with feedback on this.

Keep us posted, will you ?

Oh, by the way, since you're new to this forum, it would be nice if you would reward the correct and good answers as such, since any other readers can then easily identify the most significant answers to your question which can benefit a solution they might be looking for

Hello All,

many thanks for the warm wellcome and the ton of information you gave me.

Sorry for beeing late with my reply, but we got a huge problem in the servers which I had to take care of first.

To clarify the question of the FLARE disks:

Despite the mentioned DAEs with the 2TB SATA disks, we have another DAE which contains 6x 146GB HDDs where the FLARE resides.

So the setup of

2 DAE filled with 15x 2TB SATA drive

1 DAE filled with 8x 2TB SATA drive

does not contain any FLARE disk.

According to best practice I didn't intend to use the FLARE disks for any data despite the FLARE or in other words I will leave them exclusively for FLARE.

Summary:

When I got you all right, your recommendation would be to go for 3 RGs in the layout of 8+2 as this would be beneficial in terms of the 512 striping.

That would leave me with 4 disks unused if I understood everything right - and to overcome this the best way would be to purchase another 6 disks.

As the summary makes perfect sense to me, I don't understand why the EMC partner where we purchased offered this amount of disks (38) instead of offering 6 more in order to be able to build 4 RGs as 8+2 and have 2 spares.

I will contact our supplier and come back with feedback on this.

Thanks and kind regards

Mike

For optimal performance the 8+2 setup would be the best, but you're still free to use whatever setup you want and if this would be 10+2, this still works fine ! The strange thing is that your EMC partner didn't add any documantation on how they saw it and why they choose a setup containing 38 disks.

Hello,

when the project started, I had absolutely no idea of EMC storage, just (little) knowledge about other vendors products.

I guess that's why our supplier treated me somehow as a person with whom you won't explain/discuss the suggested hardware/config in detail.

I don't know how to translate correctly from german to english, but german sales people have a saying that would be:

(Never tell the customer more than neccessary as it may lead to unpredictable questions, additional time needed to talk him in the deal and in worst case to loss of face if he learns quickly and finds logical failures in your words  ;-)

I just asked the guys to give me a call back to discuss the reason for selling 38 disks and will keep you up to date on the answer and of course, about the way this story ends.

According to what I learned from you, my choice is 8+2 / 2HS's and thus, to purchase 6 more disks.

When the supplier does not have a really satisfying answer to why he sold 38 disks, I will ask my head of department for the confirmation to order the additional disks.

If I get them, I'll have 8+2 / 2HS's - if not, I'll stuck to 10+2 / 2HS's as this then looks as the most appealing config.

Anyway I feel better, knowing none of you sees a real problem if I  have to go for 10+2 and 2HS's - because it seems that I did at least  understand a little when reading the base documents and because if my  boss doesn not sign the purchase order for the 6 more drives it is my  only choice..

I already signed all answers where it was possible as helpful, because every one contained valuable information I am grateful for.

Thanks and regards

Mike

sure we can read it, it's just more involved ..copying and pasting into google translator