HostRaid support on FreeBSD

Søren Schmidt sos at
Wed Jun 23 15:32:39 GMT 2004

Maxim Sobolev wrote:
> Søren Schmidt wrote:
>> Maxim Sobolev wrote:
>>> Well, my knowelege about internals of software RAID controllers is 
>>> quite limited, but as long as I know they are no more than BIOS with 
>>> UI for setting up RAIDs and interface to main BIOS that hides 
>>> internal RAID structure allowing to boot off the RAID. Also they 
>>> provide some persistent storage for RAID configuration, I assume 
>>> either in some controller's NVRAM or on the HDDs. Therefore, the same 
>>> set of GEOM RAID modules can work with different software RAID 
>>> controllers (SCSI, ATA, SATA etc) if there will be a standard way to 
>>> extract this info from the controller's driver, and present it in 
>>> some common format. That's what I am talking about.
>> Well, I dont know about SCSI, but ATA/SATA always stores the metadata 
>> on disk, the controller has nothing todo with the RAID functionality, 
>> its merely a way to talk to the disks.
>> Reading the metadata off the disks and using that info to access a 
>> RAID array is easy and can be had even without docs in a few hours. 
>> Writing back the metadata in case of failures etc to retain sanity of 
>> the array is "somewhat" more difficult. Actually I'm working with this 
>> currently for new formats for ata-raid and its not trivial to get right.
> But I assume that on-disk format of this info and its location is 
> controller-dependent, so that appropriate code to read/decode this info 
> should probably be in the controller's driver.

Not exactly, its BIOS vendor depended, I know of at least a handfull of 
controller chips that can be had with at least 2 different BIOS software 
RAID's that uses different metadata.
In some cases you can even take a stack of disks thats normally on a HW 
RAID controller (like the promise sx6000) and use it on a software RAID 
setup as the metadata is the same and known by ata-raid.

At any rate, this isn't as simple as it sounds (as usual) :)


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