x86 / Linux raid

September 1st, 2009

Note not “hardware” raid cards for pc type machines actually do hardware raid.  Most of the cheaper raid controllers are doing a form of software raid that uses their driver and the system bios.

 At that point it is probably best just to use the OS to do the raid and skip the extra.

 If it states somewhere on the box  (e.g., 3w-xxxx, 3w-9xxx, aacraid, cciss, dac960, dpt_i2o, gdth, ips, megaraid, megaraid2, megaraid_mbox aka megaraid-newgen, mpt*) then there is a good chance that it is true hardware raid.

A couple of good pages

This page has a good list of what is and what is not true hardware raid.

http://linuxmafia.com/faq/Hardware/sata.html

A site that talks about what they are calling fakeraid.

http://linux-ata.org/faq-sata-raid.html

This is a good break down on what is the difference between fakeraid and hardware raid.

http://mark.kolich.com/2008/11/sata-raid-cards-linuxwindows-what-you-should-know-about-fake-raid-cards.html

I have cut and pasted this article below on the off chance this page ever disappears.  I do not do this often but on occasion I put in a link to a quality article then years later go back and it is gone.  Just want to make sure I keep this one.

 .  Be sure to check out Mark’s Blog.  It has quite a bit of real good stuff in it.  I ended up getting lost in there for a bit.  Good Stuff.

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SATA RAID Cards (Linux/Windows), What you Should Know About Fake RAID Cards

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I recently had a rude awakening regarding many of today’s RAID cards; come to find out most of today’s RAID controllers are not actually “hardware RAID” cards like you might expect.  A few days ago, I went to Fry’s and picked up a relatively cheap SATA 4-Channel RAID card (SC-SA4R12-S2) by SIIG.  I brought it home, unpacked it, and plugged it into my Linux box.  Everything was working well, except that the raw disks connected to the RAID controller were exposed under /dev:

/dev/sdc
/dev/sdd

I found this strange, because I used the controller’s Option ROM configuration utility to build a RAID-1 volume (a mirror) of the two SATA drives connected to the controller.  Assuming my RAID card drivers were installed, I was expecting to see only one device file for the “virtual RAID array” that I just created (e.g., /dev/cciss/*).  Continue reading to see what’s actually going on.

In my past life with HP-UX, I’ve worked on the HP-UX CISS Smart Array RAID driver.  Using an HP-UX Smart Array RAID solution, or a CCISS Linux Smart Array RAID solution, the OS driver only sees RAID arrays, and uses RAID-specific commands to read/write data.  The individual disks themselves are not actually exposed to the OS.  This is true hardware RAID.  If my new SATA RAID card was actually a true hardware RAID controller, I wouldn’t see both disk device files under /dev.  Instead, I would see a single device file pointing to the mirrored RAID volume.

So what exactly is going on anyways?  I thought my SATA RAID card is actually a hardware RAID card?  Turns out, it’s not.  I dug around looking for answers, and come to find out, many non-enterprise level RAID cards sold today are actually “fakeraid” cardsThese “fakeraid” cards use the OS driver and on-board flash BIOS to provide 100 percent of the RAID capability.  My cheap SATA 4-Channel RAID card (SC-SA4R12-S2) by SIIG is nothing more than a fake; it’s a bare non-RAID SATA controller that relies on the OS driver for most of the RAID operations.  That basically defeats the purpose of buying a hardware RAID card in the first place; the whole point of using a real hardware RAID controller is to offload the RAID processing from the host to the controller itself.

In any event, my SATA 4-Channel RAID card (SC-SA4R12-S2) by SIIG was only $40, so I guess I got what I paid for!  If you suspect your RAID card isn’t a real hardware RAID controller, you can check with Linuxmafia.

If you want to setup a software RAID volume on Linux, read my HOWTO guide.

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