RAID newbie question

Question

I was setting up a Poweredge 2900 at the Dell homepage and there's something about the RAID card that I don't have clear:   If I get a 'C9 Integrated SAS/SATA, RAID 1/1: Mirrored Backplane (2HDD) / Mirrored Flex Bay (2HDD)' with 3 HDD (1 80g, 2 250g) I guess this RAID card will be able to mirror (RAID 1) both 250g HDD.   But what if I get another 80g HDD? Can the RAID handle another RAID1 setup (both 80g HDD)? Or do I need another RAID card?   What does RAID1/1: Mirrored Backplane (2HDD) / Mirrored Flex Bay (2HDD) mean?   Ty in advance.

DELL-GaryS · Answer

Hi,    The SAS/SATu controller is just a SAS controller, not a H/W RAID controller, all the RAIDs in this configuration are S/W RAID, If you want to offload the main CPU of the tasking for managing the RAID, you should select the PERC 5/i controller, which is a H/W RAID.

jpp1jpp1 · Answer

I understand then, that the SW RAID could and can perform a double RAID 1 setup (80-80, 250-250) but at a CPU cost.   Is it really significant? (The PERC controller is not a bargain) :P One single PERC Controller can manage the double RAID 1 setup (4HDD) l want to build up?

DELL-GaryS · Answer

Ok the configuration you've selected with the 2 drives in the optional media bay lends itself to high performance for use with databases, as the O/S is on a seperate channel or controller than the main bay, and therefore the entire I/O thruput on the data array can be devoted to servicing the database transactions. You've indicated that you have plans to add additional drives. Here is what a H/W RAID controller buys you:   When you add the two additional drives, you can reconfigure the data array from a RAID 1 to a RAID 0+1 (what Dell calls a RAID 10) this allows you to then increase the size of the logical drive, and extend your D: partition to use the extra space. You could also opt to reconfigure into a RAID 5 that is slower on the write performance. Data 'on the fly' is protected in the case of a power loss lasting less than a day or two. You have FREE RAID management software that can be configured to send you alerts if something goes wrong with any of the drives. (I've seen a case where the customer did not have this management in place, and 3 failed drives later they're there with a down server that manages the pick and place line for a million dollar an hour in sales online pharmacy, it was out of warranty as well, and we flew in a engineer and I sent the parts and provided the technical support, all this was billed and cost them plenty, but they had it back up within 11 hours.)  The point here is you have to look at the needs of your business and factor in the future needs 2-3, even 4 years down the road. Look at the number of transactions per hour at the peak times, and how you expect that to evolve over time. What is fine now, may prove to be a bottleneck when you have 10x or 100x the business. BTW im NOT in sales :smileyvery-happy:

DELL-GaryS · Answer

The thing about software RAID, is that the O/S manages the mirroring, as long as the controller reports all 4 drives to the O/S, the OS can do the mirroring. This server could have 5 software RAID 1's.

jpp1jpp1 · Answer

I don't know, I'm getting mixed readings from what I'm googling about the SAS controller telling me either it does't support RAID or that it only supports 2 disks in raid.   Are you sure the SAS card can handle the 4HDD (RAID1 for 2 similar HDD, RAID1 for 2 similar HDD)?   Sorry about doubting :P

jpp1jpp1 · Answer

Ty now I know everything about SW RAID, one single PERC Controller (HW RAID) could also manage the double RAID 1 setup (4HDD, 2+2) l want to build up or would I need 2 PERC cards?

DELL-GaryS · Answer

Hi,    Please review the user's guide for the Perc /5 series controllers. here     The PERC 5/i has two internal channels, one for the optional bay, and the second may control the main bay, so a single controller may do the job, though for extremely demanding database applications, I've seen a configuration where they use one controller for the O/S and a second for the database, avoiding the limitation of the card to memory bus bandwidth.

jpp1jpp1 · Answer

Ty for all your help Gary. I'll check the user guide.

asaputra · Answer

Hi. I'm planning to buy PE 2900 with Raid1 for OS and Raid10 for user data. I planned to use 2x73GB SAS (Raid1) in 1x2 Flex bay and 4x300GB SAS (Raid10) in 1x8 Flex bay. Is hot spare will work if I put the hot spare drive 1x73GB and 1x300GB in 1x8 Flex bay?
Where I can find the PERC5/i installation manual for Raid 10?

Message Edited by asaputra on 12-18-200607:44 PM

pcmeiners · Answer

Two cents.... Sounds like you have resources... so for performance I would go for 6 drives in Raid 10 (industries definition of raid 10), with a hotspare... those two extra drives in the array will make a performance difference. Hotspare is important in this array. Make sure you have get a dual channel raid interface.   On the raid 1... raid 1 is  inherently  the safest raid.... the odds of two drives failing within a short period of time are near astronomical, aside from a power surge or lightning or heat affecting the server. Been doing raid for 25 years, I have never seen two drives fail in a raid 1 within a short period of time, (defined as three weeks). My suggestion would be to have a spare drive in storage when the Dell warranty ends... skip the hotspare.   Also if you intend on using this server for more than 3 years, purchase spare drives close to the 3 year mark, as obtaining new raid drives (same model), gets very difficult and expensive as time goes on. Only purchase new drives, NEVER recertified or refurbished.     As a note... I would go with the raid 1 and the raid 10 setup, this is a good setup for array servers, but do not get disappointed with the speed of the raid 1.... if you bench it, it will be no better than off a standard SAS or SCSI  adapter...high end raid adapters add no speed throughput to a 2 disk raid 1...you need the raid 1 for safety of the OS/program files and and as a separate set of spindles for tmp/log files.   Message Edited by pcmeiners on 12-20-200607:04 PM

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RAID newbie question

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