Two RAID 0 questions

[Allan Metts]

Hi all,

I want to maximize disk throughput by using Fedora's software RAID 0 on a 
plain-jane single-processor machine with 7200 RPM IDE drives.  Two 
questions come to mind:

--1-- Do I need to put the two drives on separate controllers?  Right now, 
the CDROM drive is the master drive on the secondary controller.  It seems 
cleaner to me to put the two hard drives as master & slave on the primary 
controller -- but am I sacrificing performance here?

--2-- Does RAID 0 require identical drive sizes and geometries like RAID 1 
does?  Or is it possible to span drives of differing sizes?

Thanks in advance,

Allan

[Alexandre Oliva]

On Feb 24, 2004, Allan Metts  wrote:
> --1-- Do I need to put the two drives on separate controllers?

Yes.  IDE doesn't permit simultaneous access to disks on the same
controller.  Well, you could make do with two disks on a single
controller, but performance would be terrible.
> --2-- Does RAID 0 require identical drive sizes and geometries like
> RAID 1 does?  Or is it possible to span drives of differing sizes?

If you really want RAID 0 (i.e., striping, such that both disk heads
have to move simultaneously to satisfy even small reads, which is good
for fast sequential access to big files), then you have to have
identical geometries.  If random access is more of the rule, you'll
probably be better off with linear raid, in which case the size of
individual disks doesn't matter.

I'd consider throwing LVM into the picture, since it would make the
set up far more flexible and manageable, but let's go one step at a
time :-)

[Allan Metts]

At 05:29 AM 2/25/2004, Alexandre Oliva wrote:
> Yes.  IDE doesn't permit simultaneous access to disks on the same
> controller.  Well, you could make do with two disks on a single
> controller, but performance would be terrible.

Okay, two IDE identical drives -- one will be the Master on the Primary 
controller, the other will be the Master on the Secondary controller.

Next question: Will it hurt anything to put the CDROM drive as the Slave on 
the secondary controller?  This is a no-no in the SCSI world, since I think 
it slows the entire bus down to the speed of the slowest device.  But I 
need a CDROM to actually install Fedora, and I'd rather not buy an 
additional controller....

Thanks for the help!

Allan

[Alexander Dalloz]

Am Mi, den 25.02.2004 schrieb Allan Metts um 14:46:
> At 05:29 AM 2/25/2004, Alexandre Oliva wrote:
> > Yes.  IDE doesn't permit simultaneous access to disks on the same
> > controller.  Well, you could make do with two disks on a single
> > controller, but performance would be terrible.
>
> Okay, two IDE identical drives -- one will be the Master on the Primary 
> controller, the other will be the Master on the Secondary controller.
>
> Next question: Will it hurt anything to put the CDROM drive as the Slave on 
> the secondary controller?  This is a no-no in the SCSI world, since I think 
> it slows the entire bus down to the speed of the slowest device.  But I 
> need a CDROM to actually install Fedora, and I'd rather not buy an 
> additional controller....
>
> Thanks for the help!
>
> Allan

It's no problem to have a CD-ROM as slave together with a harddrive on
the same controller. It will not slow down the whole controller.

Alexander

[Rick Stevens]

Alexander Dalloz wrote:
> Am Mi, den 25.02.2004 schrieb Allan Metts um 14:46:
> > At 05:29 AM 2/25/2004, Alexandre Oliva wrote:
> > > Yes.  IDE doesn't permit simultaneous access to disks on the same
> > > controller.  Well, you could make do with two disks on a single
> > > controller, but performance would be terrible.
> >
> > Okay, two IDE identical drives -- one will be the Master on the Primary 
> > controller, the other will be the Master on the Secondary controller.
> >
> > Next question: Will it hurt anything to put the CDROM drive as the Slave on 
> > the secondary controller?  This is a no-no in the SCSI world, since I think 
> > it slows the entire bus down to the speed of the slowest device.  But I 
> > need a CDROM to actually install Fedora, and I'd rather not buy an 
> > additional controller....

That's not true.  99% of SCSI-based CDROMs support detatched operation.
It does not slow down the bus except when the CD-ROM is transferring
data.  Bus speed is managed by the handshake between the initiator and
the target.  If you have an ultra-SCSI disk on the same bus as a CD,
the bus will run at ultra-SCSI speed when the disk and controller are
talking and at the slow speed with the CD and controller are talking.
> It's no problem to have a CD-ROM as slave together with a harddrive on
> the same controller. It will not slow down the whole controller.

That's also true.  IDE and SCSI are handled by a handshake and speed
negotiation.  The bus adapts speeds based on what's talking to what.
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- VitalStream, Inc.                       http://www.vitalstream.com -
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----------------------------------------------------------------------

[Jeff Vian]

Alexander Dalloz wrote:
> Am Mi, den 25.02.2004 schrieb Allan Metts um 14:46:
> > At 05:29 AM 2/25/2004, Alexandre Oliva wrote:
> > > Yes.  IDE doesn't permit simultaneous access to disks on the same
> > > controller.  Well, you could make do with two disks on a single
> > > controller, but performance would be terrible.
> >
> > Okay, two IDE identical drives -- one will be the Master on the Primary 
> > controller, the other will be the Master on the Secondary controller.
> >
> > Next question: Will it hurt anything to put the CDROM drive as the Slave on 
> > the secondary controller?  This is a no-no in the SCSI world, since I think 
> > it slows the entire bus down to the speed of the slowest device.  But I 
> > need a CDROM to actually install Fedora, and I'd rather not buy an 
> > additional controller....

With the newer scsi controllers it is possible to mix devices with 
different speeds on the same bus.
> > Thanks for the help!
> >
> > Allan
>
> It's no problem to have a CD-ROM as slave together with a harddrive on
> the same controller. It will not slow down the whole controller.

Statement 1 is correct
Statement 2 is not.   With IDE busses, all devices on the bus operate at 
the speed of the slowest device.  Thus if you put an ATA133 drive on a 
bus with an ATA33 cdrom, the entire bus only operates at ATA33.   They 
are improving this, and AFAICT the new SATA controllers do not have this 
limitation.
> Alexander

[Alexander Dalloz]

Am Do, den 26.02.2004 schrieb Jeff Vian um 00:35:

[ snip ]
> Statement 2 is not.   With IDE busses, all devices on the bus operate at 
> the speed of the slowest device.  Thus if you put an ATA133 drive on a 
> bus with an ATA33 cdrom, the entire bus only operates at ATA33.   They 
> are improving this, and AFAICT the new SATA controllers do not have this 
> limitation.

It only slows down for the time while the CD-ROM drive is accessed! Just
connecting a slower driver to the cable will have no affect to the
driver with the faster capability.

Alexander

[James Drabb]

On Wed, 2004-02-25 at 18:35, Jeff Vian wrote:
> Statement 2 is not.   With IDE busses, all devices on the bus operate at 
> the speed of the slowest device.  Thus if you put an ATA133 drive on a 
> bus with an ATA33 cdrom, the entire bus only operates at ATA33.   They 
> are improving this, and AFAICT the new SATA controllers do not have this 
> limitation.

That is not the case anymore.  The IDE bus will change the speed
depending on what device is currently being accessed.  I have a CD-ROM
and an ATA/100 HDD on the secondary controller and the HDD has no slow
down at all.

Jim Drabb

[Travis Fraser]

On Wed, 2004-02-25 at 08:46, Allan Metts wrote:
> At 05:29 AM 2/25/2004, Alexandre Oliva wrote:
> > Yes.  IDE doesn't permit simultaneous access to disks on the same
> > controller.  Well, you could make do with two disks on a single
> > controller, but performance would be terrible.
>
> Okay, two IDE identical drives -- one will be the Master on the Primary 
> controller, the other will be the Master on the Secondary controller.
>
> Next question: Will it hurt anything to put the CDROM drive as the Slave on 
> the secondary controller?  This is a no-no in the SCSI world, since I think 
> it slows the entire bus down to the speed of the slowest device.  But I 
> need a CDROM to actually install Fedora, and I'd rather not buy an 
> additional controller....
>
> Thanks for the help!
>
> Allan

Putting a CDROM as a Slave to one of the hard drives should not hurt performance.

I have software raid 1 set up like this and use the CDROM for loading
software, updates, etc (no net access).

Travis Fraser