Software RAID issues (was Re: Suggestions solicited, server bring up)

[Alan Johnson]

On Wed, Nov 18, 2009 at 10:44 PM, Ben Scott <dragonhawk at gmail.com> wrote:

> On Wed, Nov 18, 2009 at 8:32 PM, Alan Johnson <alan at datdec.com> wrote:
> > Would you agree that there are plenty of SATA and SCSI drivers
> > that work with most or all correctly implemented devices?
>
>   I'm talking about software (the OS, device drivers, etc.)
> interfacing to the hardware (disk controllers/host adapters).
>

Yes, I should have said controller.


> A device driver written for an Adaptec AHA-2940 card will be
> completely useless for an LSI Logic card.  The interfaces to the
> hardware are completely incompatible.
>

Certainly!  It is just news to me that there is not a third driver that
implements some shared subset of their interfaces for basic device/volume
access.

> Still, I would argue that SATA and SCSI are definitively standards ...
>
>  Yes, any proper SCSI device will talk to any proper SCSI host adapter.
>

Of course.  Just because there's no standard for the OS-controller does not
mean there is no standard.  It just applies to other parts of the system.
Silly me. =)

> ... that does not cover nearly any standard SATA device?
>
>  A SATA hard disk don't just connect directly to the microprocessor.
>

Again, I should have wrote controller, not device.  Sorry.  All that stuff
you said here is understood. =)


> > A more extreme example in another device category would be the VGA
> > standard.
>
>   There are indeed standards for video, promulgated by VESA.  There
> are no such standards for disk controllers.
>
>  Well, sort of there are.  Every disk controller has to provide an
> implementation of software interrupt vector 0x13 (INT13).  This is
> part of the BIOS specification, and is how the computer boots.  But
> once the processor switches into protected mode (or long mode, if
> you're x86-64), INT13 is no longer available.


It appears the extremity of my example is upsidedown in that the only
standard for disk controllers even more extreme than the VESA standards.
The only standard for storage controllers is a functionality so rudimentary
that is only useful for the strappiest of booting! =)  There by, not being a
valid example for my argument, but I do appreciate the mention by you for
clarity.


> So the OS has to
> provide its own device drivers.
>

Again, I'm not arguing that the OS does not have it's own device drivers,
just that it could use a generic driver for many different brands of the
same kind of devices.  I'm still fairly sure this is the case with some
categories, but I am humbled to learn it is not with SATA/SCSI, etc.


>
> > I think you got my gist.  I only mean to say that the only RAID
> controller
> > specific drivers I have had to install were for accessing the management
> > features on line.
>
>   I suspect you installed userland management software, not a kernel
> device driver.
>

This could very possibly be my point of confusion.  However, it is possible
that a RAID controller could present itself in a way that is compatible with
some more common non-RAID controller for easier integration into the
marketplace.  Have you ever seen this?  If so, that could be my point of
confusion as well.


>
> > I'm not familiar with megaraid, but this also sounds a lot like a generic
> > driver that works with devices that adhere to some standard or less
> formal
> > set of common rules.
>
>   MegaRAID is a product line from LSI Logic (formerly AMI).  Their
> "megaraid" driver works with all their SCSI RAID controller products.
> But only their cards implement that interface.  And there's a
> different driver ("megaraid_sas") for their SAS RAID controllers.
> There are a couple more drivers for their various non-RAID SCSI host
> adapter product lines.  None of them are compatible with each other.
> And that's just one manufacturer!
>

Ah, yes.  So I have gathered from other posts since writing this. =)


>
> > I think we are mostly on the same page, but are my
> > ramblings above more in line with your understanding?
>
>   I'm afraid not.  I really have to say you've got this completely
> wrong.  Sorry.
>

No worries!  Thanks for taking me to school. =)  You've laid it all out
quite nicely, as usual.  I have no reason to question anything you said here
and much of it is familar at some level and it all makes sense.  You've
helped me revive some faded bits of knowledge and connect them with some
knew bits for a much more complete understanding.  You are a great asset to
this community.
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