SATA or SCSI drives - Multiple Read/write speeds.

Mark Hahn hahn at physics.mcmaster.ca
Mon Dec 8 21:44:14 EST 2003


> > | My concern is regarding multiple disk read/writes.  With IDE, you can 
> > | wait for what seems like hours while data is being read off of the HD. 
> > 
> > nah.  it's basically just a design mistake to put two active PATA disks 
> > on the same channel.  it's fine if one is usually idle (say, cdrom or 
> > perhaps a disk containing old archives).  most people just avoid putting 
> > two disks on a channel at all, since channels are almost free, and you 
> > get to ignore jumpers.
> 
> So, admitting my near total ignorance about SATA and whether or not I
> should lust after it, does SATA perpetuate this problem, or is it more
> like a SCSI daisy chain, where each drive gets its own ID and there is a
> better handling of parallel access?

no, or maybe yes.  SATA is *not* becoming more SCSI-like: drives don't
get their own ID (since they're not on a bus).  in SATA-1 at least,
the cable is strictly point-to-point, and each drive acts like a separate
channel (which were always parallel even in PATA).  basically, master/slave
was just a really crappy implementation of SCSI IDs, and SATA has done away
with it.  given that IO is almost always host<>device, there's no real value
in making devices peers, IMO.  

yes to concurrency, but no to "like SCSI" (peers, IDs and multidrop).

> extra controller (or two).  One cable per disk if you use one disk per
> channel.

one cable per disk, period.  this is sort of an interesting design trend,
actually: away from parallel multidrop buses, towards serial point-to-point
ones.  in fact, the sata2 "port multiplier" extension is really a sort
of packet-switching mechanism...

> There is also a small price premium for SATA, although admittedly it
> isn't much.  So, in your fairly expert opinion, is it worth it?

my next 8x250G server(s) will use a pair of promise s150tx4 (non-raid) 4-port
sata controllers ;)

I don't see any significant benefit except where you need lots of devices
and/or hotswap.  well, beyond the obvious coolness factor, of course...
though come to think of it, there should be some performance, and probably
robustness benefits from Jeff Garzik's clean-slate approach.

regards, mark hahn.

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