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I am about to order a server including a DAS with 12 HDDs. However, since I haven't used such a device before I am not sure if I understood its functionality correctly.

If I connect such a device to my server:

  • Is it possible to use each HDD individually in the /dev/sdX format?
  • Is it possible to build a HW-Raid on the Server which contains HDDs in the Server and HDDs in the DAS?

Raid controller will be a MegaRAID SAS card 9280-4i4e

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2 Answers 2

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  • Is it possible to use each HDD individually in the /dev/sdX format?

Impossible to say without knowing the model and how the device is connected to the server. DAS is a very loosely used term.

Is it possible to build a HW-Raid on the Server which contains HDDs in the Server and HDDs in the DAS?

That depends on how the DAS is connected to which device. For this to work, you must connect the DAS to the ports of the hardware RAID controller and it must identify the single disks separately. If that is not possible, you can't use the disks in a hardware RAID.

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Hi Sven. It is this DAS -> ovh.co.uk/dedicated_servers/hg_2012_das.xml and this Raid-Controller -> ovh.co.uk/dedicated_servers/hg_2012_raid.xml –  Philip Mar 12 '12 at 7:52
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From what I see there, I am quite sure that it should work, but I can't guarantee that :) What I would do is to contact the provider, explain them what I plan and let them confirm that this will work with the options they provide for their servers. If they won't give you a definitive answer, you need another provider anyway, as this would demonstrate a lack of competency. –  SvW Mar 12 '12 at 8:27
    
For the record, I wouldn't add the extra drives in the system onto the same array in the DAS. The drives are very likely different speeds/sizes, and it is likely to not go well. –  Matt Simmons Mar 12 '12 at 17:06
    
@MattSimmons The drives in the DAS and in the server are identical. Do you think that it could be problematic anyway? –  Philip Mar 12 '12 at 19:04
    
It really comes back to how the drives are connected, as SvenW answered. I mean, /maybe/ hot spares, if it works? If it were me, I wouldn't do it. It makes my skin itch. –  Matt Simmons Mar 13 '12 at 19:51

If you elect to use hardware raid, you'll need to configure that raid card appropriately. If you elect to use a "JBOD" (just a bunch of disks, where each disk can be accessed on its own) you should probably use something equivalent to raid that will allow you to stripe your work across the disks, as well as provide a level of fault tolerance. Raid on a hardware card is more widely accepted as the "normal" way to do locally attached storage, however many people use ZFS to manage their disks instead of a raid card. This has its own pros and cons, and may not be appropriate for what you're looking to do.

If you choose to use standard raid, you should use the hardware raid card on your server rather than software raid. You will want to ensure that caching is enabled, and that you have a cache battery installed. It's an optional upgrade, so make sure you're getting it.

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