: : The difference between SCSI and IDE still shows up when you have
: : 2 or more drives on your system. ie.. A machine with 4 IDE drives
not really. you have to be generating heavy load on all the disks
before you really see IDE contention as a problem. of course, striping
with md is an excellent way to generate that kind of load.
: : is noticeably slower than a similar system with 4 SCSI drives.
I have lots of ide and scsi systems. the critical factor is whether
the drives are relatively new, just because density has increased so
nicely in the past year.
: : I could spend the next 500 lines detailing the technical reasons
it doesn't take that much space. here are some simple facts:
0. THERE IS NO "EFFICIENCY" DIFFERENCE! busmastering IDE works fine.
1. SCSI disks are significantly more expensive than IDE: nice SCSI
still costs around $.10/MB, whereas good IDE is around $.045/MB.
2. your motherboard already has a good IDE controller,
saving between $120 and 350 for a decent SCSI controller.
3. SCSI disks share their bus better, offer greater variety in types
of devices, and support more devices at once.
4. current commodity IDE disks sustain 8-12 MB/s, and sit on a bus that
peaks at 16 or 33 MB/s. good SCSI disks sustain 9-19 MB/s,
and sit on a bus that can be 10 MB/s, is often 20, sometimes 40 MB/s.
5. the SCSI interface is generally superior, allowing split transactions
and device-level reordering. this does NOT make a significant difference,
probably because IDE drives do readahead, write-behind and Linux does
sort the request queue.
6. good SCSI disks have _double_ the MTBF of good IDE disks. of course,
your power supply has one tenth the MTBF of even an IDE disk...
: I would be interested in those reasons. Also, does UltraDMA support
: gather reads and scatter writes?
non-ultra DMA does scatter/gather, of course. the Ultra stuff just
adds the 33 MB/s rate and support for CRC16 error detection.
in short, there are a limited number of places where SCSI makes sense,
and they do not include the typical desktop user. even a high-end user,
with, say, two 8G IBM UIDE disks might be better off without SCSI.
if you _do_ get SCSI, please don't get a controller any lower-end than
a Buslogic Flashpoint or an NCR/Sym 875-based card. they're both great
and around $120, but cheaper cards will disappoint you...
regards, mark hahn.