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SuperMicro321
Asked: 2011-01-21 01:31:22 +0800 CST

Adaptec HostRaid RAID1 1/2 of HDD capacity

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I have a server with Adaptec HostRaid controller. It has 2x 2TB drives. If I go to create a RAID0 array (striped), it will set the array's capacity to 2TB, when I go to create a RAID1 array (mirrored, and what I want to use), it sets the array's capacity to 1023GB instead of something closer to 2TB. The utility shows both drives as 1.8TB, what do I need to do to create a ~1.8TB RAID1 array?

raid1 adaptec

4 Answers

  1. This is the typical distinction in how disk manufacturers and logic systems measure disk space, disk manufacturers will assume that 1Tb is 1000 Gb where most of the other systems it'll assume that 1Tb is 1024Gb.

    Apart from this consideration it looks like your RAID card isn't performing its job, it could be highly possible that it doesn't see all the hard drive sectors, since the introduction of drives with 4k per sector (normally it was 1k per sector) most hardware devices need a new firmware update to support this, check with Adaptec to see if you can update your firmware and check if that helps, you should end up with a RAID1 with a capacity of 2Tb (or 1.8Tb, same same).

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  2. Creating a RAID 0 Array of 2x2TB drive should give you a resulting array of around 4TB, which it is not. I would suspect that your RAID controller has issues in seeing drives greater than 1TB in size. I would look for a firmware update for your controller to allow it to make use of the full capacity of all of your drives.

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  3. OK, no worries, and thanks for the drive models.

    I have to agree with the second part of lynxman's answer, that limit of 1023GB sounds very suspiciously like a programming issue with the RAID controller, and if bringing it as up-to-date as possible doesn't give you all 2TB, then I fear using software RAID will be your best bet.

    CentOS 5 supports an install onto software RAID, so you shouldn't have any problems with that.

    For what it's worth, I personally prefer software RAID to shonky hardware RAID anyway. An important element of RAID on a server is the ability to know when a drive has failed - otherwise you generally only find out when the second drive has failed, and that's not such great protection. Whilst separate RAID cards often have good Linux support, I've found that many short-lived on-the-motherboard hardware controllers have very poor Linux support, and offer you no way to check the state of the underlying drives from the running OS. mdadm will definitely give you fine-grained control over your RAID, plus the ability to use smartctl to schedule in-service tests on the drives, and hopefully to predict their eventual failures, and gracefully deal with that.

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  4. The numbers for the RAID1 setup are correct: 1 Terabyte is only 1,000 Gigabytes, etc...

    The reason you see a neat 2048Gigabytes for the RAID0 array is because your RAID card cannot cope with anything (hard disk or RAID array) bigger than 2048Gigabytes. So don't setup the RAID0 with its current BIOS/firmware: it will crash and burn at some point, a bit like a USB stick with fake capacity.

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