[conspire] Re: Dell and big disks

[Adrien Lamothe]

I've never considered using IDE RAID, for the same
reason I don't use IDE
drives in a high-performance computer. IDE data
transfer requires the CPU
for the entire transfer. A SCSI controller is actually
a separate (slave)
procesor. It receives a data request from the CPU.
then it handles the
transfer independently, using a DMA channel to send
the data to RAM;
when the data transfer is complete the SCSI controller
interrupts the CPU
to signal completion. The performance gain from SCSI
is more pronounced with
larger data transfers. I haven't seen benchmarks for
IDE vs. SCSI, but I 
suspect performace is about equal for small transfers.

Theoretically, IDE RAID could be implemented using a
slave processor just
like SCSI. The IDE slave processor would do all the
work, transferring data
directly into RAM using DMA. I'm guessing that IDE
controllers have a more
limited instruction set than SCSI, which prevents IDE
protocol from 
supporting such architecture. If true, this approach
would require extending
the IDE/ATA specification, probably not a bad thing to
do.

There seems to be a decline is SCSI use. Three factors
have contributed to this:
1. Improved IDE performance.  2. Cost considerations.
3. Use of Firewire for external devices.

IDE has gotten faster and is significantly less
expensive than SCSI. System
manufacturers who use to offer SCSI internal hard
drives in low-end servers
are now offering IDE instead, even in mid-range
systems.

Proponents of Firewall (correctly) point out that SCSI
is not hot-swappable and
Firewire is. The vast majority of external SCSI
devices were utilized by users
of Apple computers. Apple Computer was a long-time
champion of SCSI, until that
company ended up with the patent for Firewire. Many
peripheral manufacturers were
no doubt delighted that Apple started offering
Firewire instead of SCSI.
This development has led many to think that Firewire
is good and SCSI isn't.

- Adrien


--- Rick Moen <rick at linuxmafia.com> wrote:

> Quoting Adrien Lamothe (a_lamothe at yahoo.com):
> 
> > I built and configured a dual-boot Linux/Windows
> 2000 box with a SCSI
> > RAID that I striped and it doesn't have any
> problems. It boots into
> > either OS. The RAID card is an Adaptec (think it
> was a 2010S, but
> > don't remember now, it lives at a client site,
> Lawrence Berkeley Labs,
> > I can find out the model if anyone is interested.)
> 
> That sounds like a real RAID card.  To my knowledge,
> all Adaptec RAID
> controllers except the 1200 series (which use a
> Silicon Image chip, not
> an Adaptec one) are real RAID.  That means that the
> OS and its drivers
> don't see, and don't have to manage, individual
> disks, but instead see 
> RAID volumes (which you typically establish
> separately using either the
> RAID BIOS program or a manufacturer-furnished
> utility).
> 
> "RAID" chips built into motherboards are inevitably,
> on grounds of
> minimising expense, what on comp.os.linux.hardware
> are termed "fakeraid"
> controllers.  I like that term so much that I've
> adopted it in my page
> on Serial ATA considerations for Linux:
> 
> "Serial ATA" on http://linuxmafia.com/kb/Hardware/
> 
> > Perhaps the RAID controller on the computer in
> > question can't boot when striped? Its worth
> trying.
> 
> 1.  I'm pretty sure we're talking about mirroring
> (RAID1) not striping
> (RAID0).
> 
> 2.  The fact that a motherboard-embedded chipset
> purports to do its own
> RAID (and, inevitably, turns out to be fakeraid)
> doesn't mean you need
> to use that functionality.  Which is a good thing,
> since fakeraid
> implementations inevitably suck compared to the same
> functionality in
> Linux software RAID (the "md" = multiple device
> driver).
> 
> 3.  I vaguely recall that booting from a Linux
> software-RAID mirrored
> volume is tricky, in some fashion or other.  When in
> doubt on such
> matters, start at your favourite mirror of the Linux
> Documentation
> Project and browse the HOWTOs.  (If you don't have a
> favourite, go to 
> http://www.tldp.org/ .)  Notice the mnemonic:  The
> Linux Documentation
> Project, tldp.org .  And, if you can't remember
> that, you can always 
> google for "Linux Documentation".
> 
> Soon, you find yourself in the alphabetical list of
> all HOWTOs:
> http://www.tldp.org/HOWTO/HOWTO-INDEX/howtos.html
> 
> And that leads you, in this case, to the
> Software-RAID-HOWTO .
> It has a section about issues involved with "Booting
> on RAID" (section 7.3) 
> and similar matters.
> 
> -- 
> Cheers,                 There are 10 kinds of people
> in the world, those who 
> Rick Moen               know ternary, those who
> don't, and those who are now 
> rick at linuxmafia.com     looking for their
> dictionaries.  -- Ron Fabre
> 
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>
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> 



		
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