[Discuss] PCI RAID card for Linux?

[Ed Fletcher]

Deryk Barker wrote:
> Jim Roepcke wrote:
>>
>> IIRC, if I put an ATA hard drive on the same channel as the ATA CD-
>> ROM drive the performance of the hard drive will suffer.  Is that 
>> true?  Should I be worried about that or is that just a myth/bunk?
> 
> It used to be the case that CD-ROMs would only do 16-bit I/O and the IDE
> channel it was on would therefore only do 16-bit, which was bad news for
> any hard drive on the same channel.
> 
> Whether this is still true, I have no idea.
> 
The problem with IDE is that when a request is made to a device on the
bus, no more requests can be issued until the first request has been
answered.  So if you have a cd-rom and a hard drive on the same bus and
send a read request to the cd-rom, you cannot access the data on the
hard drive until the cd-rom has finished answering the request.  If you
are using the cd a lot, it will have a large effect on access times to
the hard drive on the same IDE cable.

Note that if you are not using the cd-rom, then it will have no effect
on the speed of access to the hard drive.  So it all depends on how much
you use the cd drive.

SCSI doesn't have this problem because requests can be interleaved in
that data and/or new requests can be sent anytime the bus is idle.  That
is, you can send a read request to device 1 and while that device is
searching for the data, you can send a read request to device 2.

Since SATA only has one device per cable, it doesn't have this problem
either.  Unless having two ports on one controller imposes any limits,
that is.  I don't know enough about SATA to say.

If you really want to separate your raid array from your cd, I can tell
you from personal experience that Promise IDE controllers work great
with software raid in Linux.  They're not too expensive either.

HTH,
Ed
-- 
Ed Fletcher
ed at fletcher.ca

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