Re: ide-scsi with 2 drives
On Thu, 30 Dec 2004, Ron Johnson wrote:
> The thing is that, unlike SCSI, only one device can be using an
> IDE bus at any one time.
a common misconception ...
electrically ...
only one ide disk can drive the signals on the ide cable at any
time
similarly, even scsi disks, only one disk can be driving
the cable at any time
==
== if 2 scsi disks tries controlling the signals on the cables at
== the same time, you'd have a major short circuit
==
-----------
ide disks controllers can pretend to support host swap
but the device drivers does NOT support it
scsi disks can do hot swap and is supported by scsi drivers
----------
ide specs allow for 18" max cable length at a specific impedence
and resistance and operating environment
scsi specs allow for 6'(?) max cable length at a specific
impedence and resistance and opertating environment
----------
with ide .... you have 2 disk per cable
with scsi ... you can have 15 or 32 disks per cable ..
where all 15 disks have to share one cable and WAIT
- hopefully, the disk controller has enough buffer to
allow the cpu to merrily continue without waiting
- good thing nobody makes a 32 connector scsi cable
or a 255 connector firewire cable :-)
disk driver ...
in the old days ... ide disks had 2MB disk buffer
but todays ide disks are also 8MB buffers
in the old days ... scsi disks had 8MB disk buffer
and hasn't changed in 8(?) yrs
- in the old days, ide did not have tagged queing
but in todays ide controllers, it too has tagged queuing
depending on the motherboard chipset
- scsi controllers always had tagged queing .. so the system can
pretend the data was "really" written to the disks and merrily
move along
- in the old days ... ide disks rotated at 5400, 7200,10K
and still have some catch up
- in the old days ... scsi disks roated at 7200, 10K, 15K
and runs "hot"
- how fast can you write was a big deal in the old days
8MB/sec vs 16MB/sec
16MB/sec vs 40MB/sec
33MB/sec vs 80MB/sec
66MB/sec vs 120MB/sec
100MB/sec vs 160MB/sec
133MB/sec vs 320MB/sec
..sata.. vs iscsi
..iata.. vs what's-next
transfer speeds ...
ata-133 is supposed to be 133MB/sec
but your real transfer speeds is highly dependent upon
which motherboard and which chipset
- ata give up chasing scsi and changes name to sata
ultra-scsi-160 is supposed to be 160MB/sec
ultra-scsi-320 is supposed to be 320MB/sec
= since ide and scsi doesnt run at the same spec'd speeds,
= one can only test for how close to the marketing numbers
= it can achieve on the same mb/cpu/mem
more fun ...
c ya
alvin
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