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A couple of RAID questions. I think I know the answers, but just wanted to double-check since I am about to play will several years worth of personal data.

Do I understand correctly that:

  1. If I have a RAID-1 (mirror) setup, then I cannot use individual disks on their own, as standalone drives. In other words I cannot stick RAID'd disk into an USB enclosure and expect the OS to recognize it?

  2. If I have a RAID setup, I cannot expect a RAID controller of one manufacturer to recognize disks that were set up with a controller of another?

Both questions basically boil down to this - if the format of individual RAID-1 disks is somehow standardized, and how a RAID-specific control data is stored on a disk (if any).


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up vote 3 down vote accepted

Both answers depend on the type of the RAID controller. For example, in my previous job, we successfully recovered data from a RAID-1 drive by pulling it out, putting it into an external USB enclosure, and plugging it into another computer, which recognized it and read it as if there was nothing special or unique about it. (This was an EXT3 file system pulled from an Ubuntu Linux server with hardware RAID and plugged into an Ubuntu Linux workstation with no RAID configuration at all.) On the other hand, we also had RAID-1 drives that were completely unreadable ("Would you like to format this drive?" "NO!!!") when connected to anything but the RAID controller they were initially set up on.

I've heard from friends and colleagues who are also in sys admin roles of success stories of pulling entire RAID arrays and successfully setting them up (with no loss of data) on a different RAID controller from an entirely different manufacturer. My own experience in this arena, however, is that one RAID controller cannot read disks set up for/by another RAID controller. YMMV.

Bottom line: If your goal is portability of your RAID disks, then you have two choices:

  1. Pick and stick with one RAID controller. Don't use your disks with any other controller.
  2. Use software RAID -- I'm a huge fan of MD on Linux, however software RAID of course lacks the performance you'd get from the dedicated hardware of a physical RAID controller.
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+1 because it's the right answer: depends on the RAID controller, most won't let you. About MD, it's true it does let you use any kind of block access, but its doesn't 'lack the performance'. It will happily satuate most disks bandwith. The only drawback against a good hardware RAID is CPU overhead, but that's only significant in extreme cases – Javier Apr 11 '11 at 17:33
Got it, thanks. I really wish there was a simpler form of RAID-1 whereby each disk would be formatted in native NTFS just as if it was connected to a regular controller. And whatever control data that is needed would be stored in an enclosure itself (on a flash). Basically the idea is to be able to take the disk out of the array, stick it into a laptop/desktop and have the OS recognize it. – Pedro San Lukas Apr 11 '11 at 17:41
@Pedro I think that must be how the controller worked where we were able to extract and use the disk on its own -- there was absolutely no indication on the disk that anything but the file system and data was there at all. @Javier Software RAID incurs CPU overhead and is limited by the I/O of the bus -- sending e.g. twice as much data through it in a 2-disk RAID-1 setup. While I agree with you that MD is on par with hardware RAID in practical terms, the reality is that hardware will be superior in true high-performance scenarios. – Kromey Apr 11 '11 at 18:12

1: depends. MOST lieky not with a good controller as it will use at least a partition at the beginnign to store configuratio ndata.

2: no. Simply no. No standard. Moving cards between controllers is standard these days, though, by most serous manufacturers. Every disc stores all relevant information in a partition.

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Thanks, appreciate the reply (can't upvote you though, because I am not worthy of this godly action yet). – Pedro San Lukas Apr 11 '11 at 17:49

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