Brian J. Murrell <brian@xxxxxxxxxxxxxxx> wrote: > I understand that dmraid is just some kind of interface to the > _software_ raid that is provided on a number of SATA interface cards > such as the Promise and Adaptec cards. Well, to be exact, dmraid creates device-mapper tables depending on the on-disk-layout of various proprietary Software-RAID solutions. It does neither co-operate with the respective BIOS-subsystems nor does it manage the respective on-disk metadata to mark disk-failures, change layouts etc. > Reading http://ata.wiki.kernel.org/index.php/SATA_RAID_FAQ I get the > message loud and clear that this is not real hardware RAID but rather > it's done in the BIOS on the card. Most of the proprietary Software-RAID solutions out there are implemented in software only. All the "BIOS" contains there is some very basic support to initialize and boot off such disk-conglomerates. > The message there seems to be, if you are going to do it in software > anyway, why not just skip the IO card RAID and just use MD? Yes, that's the much cleaner approach. And, for Linux, the more stable one as well - since dmraid doesn't manage drive-faults etc. anyways as noted above. > The one reason I could think of for using the BIOS provided RAID would > be to reduce the data needing to traverse the PCI bus. With host-raid > (i.e. MD) every write to disk needs to actually be written over the PCI > bus twice, once for each disk, right? Most of the proprietary Software-RAID solutions out there don't do this, especially not the low-cost ones that only support RAID0/RAID1/RAID10. Some solutions that support RAID5 (not all of them, though) provide on-chip XOR engines - I would suspect those to be able to spread writes after the PCI(e) bus. > IIUC, when you have a BIOS RAID configured, even though there are two > disks, the operating system only sees one. This would seem to support > the theory that the write only goes to the controller once and it takes > care of the mirroring, but I will defer to your experience. Well, usually the drivers are the ones that hide single devices for ease and convenience, not the controllers. regards Mario -- Tower: "Say fuelstate." Pilot: "Fuelstate." Tower: "Say again." Pilot: "Again." Tower: "Arghl, give me your fuel!" Pilot: "Sorry, need it by myself..." -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
