Dell PowerEdge 2800 – Explanation of RAID Hard Drive Configurations & the reasons for a choice

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I have been researching the purchase of a new server for our company. We are a non-computer manufacturing company. Currently we have a Mirror disk configuration. I want to utilize RAID5, stripping with parity, and Hot swap capability on the new server. The server of interest to me is the Dell PowerEdge 2800. Where I have had difficulty is to determine what the provided options really indicate and what best fits our environment. The hard drive configuration options in the Select Components leave me confused on what I am selecting and why would one be chosen over another. Also, what is meant by the reference to a media bay? The options available offer, in most cases, 3 flavors which are RAID 0, RAID 1 and RAID 5. So, I will just list the categories of RAID: 1. Drives attached to embedded PERC4ei {I learned the acronym means PowerEdge Expandable Raid Controller, "New embedded ROMB (the 'i'), U320 (hence the 4) and PCI-Express (the 'e'), which happens to be based on LSI megaraid (yes, uses megaraid2 driver)"} 2. AR5/N Add-in card RAID 5, No Drives in Media Bay, 2 drives required. What is AR5/N? And should not there be 3 minimum of 3 drives? 3. Drives attached to PERC4ei RAID5. If this is an add-in card, why would I choose it versus the embedded PERC4ei? 4. RAID 5 Backplane Drives/ RAID 1 Media Bay Drives; 3 drives required. Why would I want a backplane? 5. Split Backplane, Drives attached to PERC4ei, RAID 1, 5; 5 drives required. Why would I want a split backplane? Why is RAID1 & RAID 5 listed? 6. Split Backplane, Drives attached to RAID card, RAID 1, RAID 5; 5 drives required. Why is RAID1 & RAID 5 listed? My objective for this server's configuration, is as much redundancy as I can obtain and hot swap ability of the hard drives.

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RAID1 is maximum redundancy, so why do you want to switch to RAID5. RAID0 is striping with no redundancy. There are cheaper solutions than RAID1 but no better. RAID5 provides protection of data in the event of an HDD failure, but the rebuild operation really hits performance badly. RAID1, of course, needs an even number of HDD, unless one is kept as hot spare. If RAID1 gives you enough space (and a spare) then don’t consider anything else. In low-end RAID5 implementations (e.g., direct attached disk in Dell server) there is a significant write performance overhead from the parity striping (which provides the availability protection). There is no such thing as a free lunch (TINSTAFL). In high-end EMC, HP, Sun, and HDS storage directors used in large enterprise SANs back-end micro-processors and large caches are used to avoid read-back/write-back RAID5 parity write overhead, but RAID5 on internal disks may save (a very little) money more often than they make sence.

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  • Rlmink
    Thanks for the explanation regarding the performance issue of RAID1 versus RAID5. The information adds another element of consideration. My objective with my query, is to also understand the hardware options that Dell is providing. Before a dialogue starts with a Dell account rep, regarding the RAID hardware options, I wanted to hear from a independent source. My major objective in my query is information specific to Dell's hardware RAID options. Thank you for the facts you have sent me.
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  • TooManyHats
    In response to your question about why would you want to use an add on card and not the imbedded controller. These add on SCCI RAID cards allow you to attach to multiple arrays and carry there own memory and processing power. This will help alleviate and remove the overhead during heavy writes. This also adds a layer of redundancy in that if the machine has a complete failure and losses power the batteries on the RAID card can keep the data cashed in memory until power is restored. When power is restored the data can finish being written to the drives. I am not familiar with the Dell RAID cards so check them out and see what they actually offer. You can compare them to the Compaq/HP line RAID controllers to get an idea. Some even offer data protection even if you lose 2 physical drives in a RAID 5 array.
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  • Rapier57
    There are two kinds of RAID backplanes offered by Dell. The one, single backplane, is used if you do not plan to set up more than the one internal array. The split backplane is used if you plan to have more than one array (say, one internal and one external). How you select the drives should be determined by the role the server plays in your network. If this is a mission critical server, you may want to configure three or more drives in the system as a single RAID5, then build two containers. One container for the boot drive and the second partition for the data drive. This helps to insure that the server can stay in operation with a failed drive until a replacement arrives. Then it is a simple matter of a hot swap and rebuild. On my own orders, I set up the RAID5 configuration with the PERCs, with five or more drives. We do the array container configuration at our location, using the adapter configuration. One container is configured with the first two drives in the array as a mirror set RAID 1. The OS is installed on this. It provides a redundant, though not hot-swappable drive array. If one of the drives fails, the mirror is broken and the system can continue to run until a replacement drive arrives. The mirror has to be reconstructed with the new drive, therefore the system must be restarted again. The rest of the drives are configured in a RAID5 container and set up as drive D: (or E:, etc). This provides a hot-swappable array that allows failed drives to be replaced without taking the system down. Rebuild is a performance hit, but as long as you have "right-sized" the array (purchase drive capacity at 2 to 3 times the expected space required) it isn't severe. Also, keep in mind that drive I/O is improved with more spindles in the array.
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