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I'm setting up a new NAS/SAN system with RAID5, and I was wondering if going the software or hardware RAID way because I had this question in mind:

If my hardware-RAID card fails, will I need to substitute by one exactly the same, same brand is enough or are RAID5 setups incompatible between different cards?

My guess is that they are not compatible, and I'm trying to get the less downtime possible in case a piece of my hardware fails...

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2 Answers 2

up vote 6 down vote accepted

Cards from different manufactuers are typically not compatible, although different cards from the same manufacturer usually are. There is no particular standard format for RAID metadata that is compatible across RAID controllers and software RAID implementations.

If you have an adaptec (for example) card and get a different model of adaptec card, the new card will almost certainly mount the old array. In some cases the same also applies to SAN equipment and RAID controllers from the same manufactuer - Mylex DAC-FFX controllers and ExtremeRAID 3000 cards used to do this (in fact an ExtremeRAID 3000 was essentially a DAC-FFX on a PCI card), and HP Smartarray 1000 & 1500s will also mount arrays transferred from HP direct attach controllers.

This is usually a deliberate policy on the part of the manufacturer to allow substitution of current parts if an older model is no longer available in stock. It also helps with upselling existing DA customers onto entry level SAN equipment by easing the migration path - just pop the disks into the SAN and mount the volumes off the SAN.

Note, however that OEM contracts and mergers and acquisitions mean that manufacturers may have several incompatible product lines. For example:-

  • Adaptec bought Eurologic and sold Eurologic SAN equipment for a while. Eurologic SANs have Mylex RAID controllers.

  • Adaptec also purchased ICP Vortex, so some adaptec branded RAID controllers may not be compatible with others in this respect.

  • LSI also purchased Mylex at one point and sold ExtremeRAID controllers for a while, but have their own line of host-based RAID controllers. The final dissolution of Mylex was quite complex with bits going to Xyratex (the biggest manufacturer of disk array hardware you've never heard of making stuff branded by people you have) as well.

  • Intel, Dell PERC, IBM ServerRAID controllers and some HP parts are often rebadged items made by a third party (ICP Vortex, Adaptec and LSI controllers often pop up with other brands). The OEM ones tend to have custom firmware but may still represent multiple, incompatible product lines. However, branded kit of this sort tends to have specific part numbers so you can re-order compatible replacements .

Do your homework. Check with the manufacturer, avoid anyone who gives you a blank stare and make sure you know which models go with each other in this respect.

Note Linux and Unix have good SW RAID facilities but software RAID on Windows is poo. If you're using Windows then always go for hardware RAID. Ebay is your friend if you get sticker shock at the price of new kit. Make sure you get a cache battery for it - often you can get them quite cheaply off ebay - again, find the part number and hunt.

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As usual the problem is that "almost certainly". I'm sure if I have any problem in my big HP server, or with good Adapter controller, that I won't have any problem when substituting it. But when dealing with cheaper devices, like a smaller Promise or Dawicontrol card, I'm not sure if they work that good, and, also, I can check with the manufacturer, but if it's not written on the papers... I'm not confident... –  Andor Jun 22 '10 at 14:20
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I've never used Promise controllers with HW RAID, so I have no personal experience of them. However, you can get an Adaptec or LSI card for $1-200 off Ebay so I tend to do that if I want RAID on the cheap (from time to time I have occasion to make a data warehouse development box for myself or a client so I've done this a few times now). I recommend Adaptec or LSI for this. Look at the specs on the web site and search by model number on Ebay. –  ConcernedOfTunbridgeWells Jun 22 '10 at 14:25
    
Adding to ConcernedOfTrunbridgeWells, HP's line of SA-Pxxx controllers are also generally available a reasonable prices on eBay. List of current HP SA Controllers: h18000.www1.hp.com/products/servers/proliantstorage/… –  Chris S Jun 22 '10 at 14:39

Although many manufacturers use the same basic chipsets to create their various RAID cards (i.e. some HP and Dell controllers are very similar) I certainly wouldn't trust in their similarity to move an array from one manufacturer to another - not with hardware RAID. As CoTW states most HP controllers are indeed backwardly compatible. If you use software RAID then this will be much less of an issue but of course you put more load on your CPUs doing this.

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I think I will have no problem giving CPU power for the RAID control... Dual/Quad Core CPUs are quite powerful this days, and this isn't going to be a "high performance" NAS, but a two to four users code repository and maybe a webcache for them. I'm more interested on being safe that being bleeding fast. Of course, we will be checking an UPS also... –  Andor Jun 22 '10 at 14:16

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