[lug] software RAID

[Hugh Brown]

On Sat, 2006-02-11 at 16:56 -0500, Dean Brissinger wrote:
> On Friday, February 10, 2006 10:20 PM D. Stimits wrote:
> 
> > Jeff Schroeder wrote:
> >> Hugh wrote:
> >>> read something that said putting them on the same ide interface is
> >>> worse than bad.  If so, is it only performance related or do
> >>> terrible things happen (guess who put both drives on ide1).
> >> 
> >> 1) Performance, as you said.  Since you're writing to both drives in
> >> a RAID-1 configuration, you'll be crowding the bandwidth on the IDE
> >> channel.  Using two channels will allow you to have greater speed.
> >> 
> > Worse than that: IDE does not allow two drives simultaneous access.
> > Maybe SATA is better, but I don't know. In terms of SCSI, there is a
> > fast burst then the drive disconnects from the bus. SCSI has no
> > problem dealing with maybe 7 drives on one cable (except perhaps for
> 
> It's very bad:
> 
> In the case of a mirror putting two devices that would be activated at
> the same time will dead-lock many systems.  If not dead-lock it will run
> very very slow.  The reason being IDE controllers do not provide a cache
> for events that would allow both devices to alternate control of the bus
> (like SCSI does).  Take the case you put a Zip drive and a system drive
> on the primary IDE bus as master and slave.  Everything will be fine
> until you copy a file from the system drive to the zip drive.  At which
> point the system will a) need swap, b) need dedicated i/o to the hdd for
> read, and c) need dedicated i/o to the zip.  The controller will refuse
> two and the end result is a deadlock after one or two (lucky) cached
> writes.  The solution is to add IDE busses.  Check out some IDE/PCI
> adapters.  Many conflict w/ the system bios in old hardware though.
> 
> SATA does not address this.  SATA raids are built with one SATA bus per
> disk and combined with a fibre channel or SCSI backplane.  You can
> actually get chassis now that can interchange SATA and SCSI disks in the
> same slot because of this design.
> 
> Also, modern SCSI can talk to many more drives than 7 on a single chain.
> All SCSI/FibreChannel hard drives have a tuneable option for how long
> they hold the bus after they are done talking.  In build-your-own
> environments it is required to tune this to zero to prevent bus
> overruns/cache overflows.  Though, I don't recommend building such a
> thing home-grown.  If you need something like that go buy a JBOD chassis
> with a solid silicon backplane for it.  ;-)



Thanks to everyone for their input.  I just ordered a cheap 2port 3ware
card to allay my fears.

Hugh