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.
/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" cards. These "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.
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" cards. These "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.
Did you find this post helpful, or at least, interesting?